Improve FWRETRACT config options and commentary

Marlin/Configuration_adv.h

...mple_configurations/AlephObjects/TAZ4/Configuration_adv.h

Marlin/example_configurations/Anet/A6/Configuration_adv.h

Marlin/example_configurations/Anet/A8/Configuration_adv.h

...n/example_configurations/BQ/Hephestos/Configuration_adv.h

...example_configurations/BQ/Hephestos_2/Configuration_adv.h

Marlin/example_configurations/BQ/WITBOX/Configuration_adv.h

Marlin/example_configurations/Cartesio/Configuration_adv.h

Marlin/example_configurations/Felix/Configuration_adv.h

...le_configurations/Folger Tech/i3-2020/Configuration_adv.h

...ample_configurations/Infitary/i3-M508/Configuration_adv.h

...in/example_configurations/Malyan/M150/Configuration_adv.h

Marlin/example_configurations/RigidBot/Configuration_adv.h

Marlin/example_configurations/SCARA/Configuration_adv.h

Marlin/example_configurations/TinyBoy2/Configuration_adv.h

...example_configurations/Velleman/K8200/Configuration_adv.h

...example_configurations/Velleman/K8400/Configuration_adv.h

...igurations/delta/FLSUN/auto_calibrate/Configuration_adv.h

...onfigurations/delta/FLSUN/kossel_mini/Configuration_adv.h

.../example_configurations/delta/generic/Configuration_adv.h