Commit 689231df changed the default step_pulse_duration to 100ns for
TMC drivers that are configured in UART/SPI mode, but it failed to
document that change in Config_Changes.md and Config_Reference.md.
Update those documents to reflect that change.
Update the code so that a user may specify a custom
step_pulse_duration even on TMC drivers that are configured in
UART/SPI mode. This may be useful for drivers that use a level
shifter on the step pin that requires more than a ~300ns to detect a
level change.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
520273e5ab