drm/i915/doc: Document where to implement register workarounds

Extend the existing documentation in gt/intel_workarounds.c to make it
clear which functions register workarounds should be implemented in
according to their types.

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118155249.41551-2-gustavo.sousa@intel.com