saul/doc: Warn about and deprecate saul_reg_rm
saul/doc: Warn about and deprecate saul_reg_rm

saul_reg_rm can not be used safely unless in tight coordination with the
application. This points out the issue, puts the onus of making sure
nobody calls vanishing registrations on the device driver and deprecates
the call (as the requirements for correct usage are practically
impossible to fulfull for a generic driver).

kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,basilfx/RIOT,OlegHahm/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,basilfx/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,basilfx/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,basilfx/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,ant9000/RIOT,basilfx/RIOT,kYc0o/RIOT,kYc0o/RIOT,kYc0o/RIOT