drm/nouveau/doc: update the thermal documentation

Changes:
 - Change the maintainer's address (the labri address will expire soon);
 - Drop the note about not all families supporting all fan modes;
 - Add a note about the reported RPM not being accurate when driven outside
   the vbios-defined PWM range.

Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>