delta: max z-accel and speed ratio calculation for deltas (#4689)

Adding a possibility to use a lower z- acceleration.

Signed-off-by: Martin Malmqvist <volcomosq@gmx.com>

docs/Config_Reference.md

@@ -250,6 +250,11 @@ max_z_velocity:
 #   maximum speed of up/down moves (which require a higher step rate
 #   than other moves on a delta printer). The default is to use
 #   max_velocity for max_z_velocity.
+#max_z_accel:
+#   This sets the maximum acceleration (in mm/s^2) of movement along
+#   the z axis. Setting this may be useful if the printer can reach higher
+#   acceleration on XY moves than Z moves (eg, when using input shaper).
+#   The default is to use max_accel for max_z_accel.
 #minimum_z_position: 0
 #   The minimum Z position that the user may command the head to move
 #   to. The default is 0.