Documentation issue regarding M917

[ampex_fhm]

In the docs, the section about M917 states

Some motor drivers (e.g. TMC2660)

but the TMC2660 does not support the standstill motor current reduction as evident here:

https://github.com/dc42/RepRapFirmware/blob/v2-dev/src/Movement/StepperDrivers/TMC2660.cpp#L896

This should be changed to avoid confusion.

edit: Also the docstring at the top of the TMC22xx files read TMC2660.cpp and ExternalDrivers.h respectively

[dc42]

My intention was to implement standstill current reduction in firmware, but I haven't done that yet.

I will correct the docstrings in the source files at the next commit.