docs: Update Status_Reference.md with information on accessing coordinates

Update the docs with information on how to access the "extra axes" of
coordinate values.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Status_Reference.md

@@ -214,17 +214,16 @@ The following information is available in the `gcode_move` object
 (this object is always available):
 - `gcode_position`: The current position of the toolhead relative to
   the current G-Code origin. That is, positions that one might
-  directly send to a `G1` command. It is possible to access the x, y,
-  z, and e components of this position (eg, `gcode_position.x`).
+  directly send to a `G1` command. This value is encoded as a
+  [coordinate](#accessing-coordinates).
 - `position`: The last commanded position of the toolhead using the
-  coordinate system specified in the config file. It is possible to
-  access the x, y, z, and e components of this position (eg,
-  `position.x`).
+  coordinate system specified in the config file. This value is
+  encoded as a [coordinate](#accessing-coordinates).
 - `homing_origin`: The origin of the gcode coordinate system (relative
   to the coordinate system specified in the config file) to use after
   a `G28` command. The `SET_GCODE_OFFSET` command can alter this
-  position. It is possible to access the x, y, and z components of
-  this position (eg, `homing_origin.x`).
+  position. This value is encoded as a
+  [coordinate](#accessing-coordinates).
 - `speed`: The last speed set in a `G1` command (in mm/s).
 - `speed_factor`: The "speed factor override" as set by an `M220`
   command. This is a floating point value such that 1.0 means no
@@ -236,6 +235,10 @@ The following information is available in the `gcode_move` object
   coordinate mode or False if in `G91` relative mode.
 - `absolute_extrude`: This returns True if in `M82` absolute extrude
   mode or False if in `M83` relative mode.
+- `axis_map`: Provides a mechanism for finding the coordinate
+  component for a given G-Code id that is used in `G1` commands. See
+  the [Accessing Coordinates](#accessing-coordinates) section for
+  details.
 
 ## hall_filament_width_sensor
 
@@ -361,7 +364,8 @@ The following information is available in the `motion_report` object
 (this object is automatically available if any stepper config section
 is defined):
 - `live_position`: The requested toolhead position interpolated to the
-  current time.
+  current time. This value is encoded as a
+  [coordinate](#accessing-coordinates).
 - `live_velocity`: The requested toolhead velocity (in mm/s) at the
   current time.
 - `live_extruder_velocity`: The requested extruder velocity (in mm/s)
@@ -550,9 +554,8 @@ objects (eg, `[tmc2208 stepper_x]`):
 The following information is available in the `toolhead` object
 (this object is always available):
 - `position`: The last commanded position of the toolhead relative to
-  the coordinate system specified in the config file. It is possible
-  to access the x, y, z, and e components of this position (eg,
-  `position.x`).
+  the coordinate system specified in the config file. This value is
+  encoded as a [coordinate](#accessing-coordinates).
 - `extruder`: The name of the currently active extruder. For example,
   in a macro one could use `printer[printer.toolhead.extruder].target`
   to get the target temperature of the current extruder.
@@ -560,8 +563,8 @@ The following information is available in the `toolhead` object
   "homed" state. This is a string containing one or more of "x", "y",
   "z".
 - `axis_minimum`, `axis_maximum`: The axis travel limits (mm) after
-  homing.  It is possible to access the x, y, z components of this
-  limit value (eg, `axis_minimum.x`, `axis_maximum.z`).
+  homing. This value is encoded as a
+  [coordinate](#accessing-coordinates).
 - For Delta printers the `cone_start_z` is the max z height at
   maximum radius (`printer.toolhead.cone_start_z`).
 - `max_velocity`, `max_accel`, `minimum_cruise_ratio`,
@@ -571,6 +574,10 @@ The following information is available in the `toolhead` object
 - `stalls`: The total number of times (since the last restart) that
   the printer had to be paused because the toolhead moved faster than
   moves could be read from the G-Code input.
+- `extra_axes`: Provides a mechanism for finding the coordinate
+  component for extra axes available in standard `G1` type move
+  commands. See the [Accessing Coordinates](#accessing-coordinates)
+  section for details.
 
 ## dual_carriage
 
@@ -627,3 +634,29 @@ The following information is available in the `z_tilt` object (this
 object is available if z_tilt is defined):
 - `applied`: True if the z-tilt leveling process has been run and completed
   successfully.
+
+## Accessing Coordinates
+
+Some status fields provide a "coordinate". For macro users these
+fields may be accessed by component name
+(eg,`{printer.toolhead.position.x}`), where the component name may be
+"x", "y", or "z".
+
+For developers using the Klipper API Server these fields are
+transmitted as a list - for example: `{"toolhead": {"position": [1.0,
+2.0, 3.0, 7.3, 19.2]}}` . The first three components of the list
+correspond with the x, y, and z axes.
+
+A coordinate will typically have at least 3 components (x, y, and z),
+however there may also be additional components. Care should be taken
+when accessing any of these additional components as the ordering and
+number of components may change at run-time.
+
+One may use `{printer.gcode_move.axis_map}` and/or
+`{printer.toolhead.extra_axes}` to determine the number of components
+and the ordering of components. For example, to access the "E"
+component one could use
+`{printer.toolhead.position[printer.gcode_move.axis_map.E]}`. Or, if
+one wanted to find the component associated with the "extruder"
+object, one could use
+`{printer.toolhead.position[printer.toolhead.extra_axes.extruder]}`.