toolhead: Replace junction_deviation with square_corner_velocity

The junction_deviation configuration parameter has a number of quirks
that make it difficult to configure.  Replace it with a
"square_corner_velocity" configuration parameter.

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

docs/FAQ.md

@@ -220,7 +220,7 @@ TMC driver. Trinamic has indicated that this could occur if the driver
 is in "stealthChop mode" and an abrupt velocity change occurs. If you
 experience this problem during homing, consider using a slower homing
 speed. If you experience this problem in the middle of a print,
-consider using a lower junction_deviation setting.
+consider using a lower square_corner_velocity setting.
 
 ### When I set "restart_method=command" my AVR device just hangs on a restart
 

docs/G-Codes.md

@@ -97,9 +97,9 @@ The following standard commands are supported:
   /tmp/heattest.txt will be created with a log of all temperature
   samples taken during the test.
 - `SET_VELOCITY_LIMIT [VELOCITY=<value>] [ACCEL=<value>]
-  [ACCEL_TO_DECEL=<value>] [JUNCTION_DEVIATION=<value>]`: Modify the
-  printer's velocity limits. Note that one may only set values less
-  than or equal to the limits specified in the config file.
+  [ACCEL_TO_DECEL=<value>] [SQUARE_CORNER_VELOCITY=<value>]`: Modify
+  the printer's velocity limits. Note that one may only set values
+  less than or equal to the limits specified in the config file.
 - `SET_PRESSURE_ADVANCE [EXTRUDER=<config_name>] [ADVANCE=<pressure_advance>]
   [ADVANCE_LOOKAHEAD_TIME=<pressure_advance_lookahead_time>]`:
   Set pressure advance parameters. If EXTRUDER is not specified, it