drm/xe: Expand XE_REG_OPTION_MASKED documentation

drm/xe: Expand XE_REG_OPTION_MASKED documentation

Expand documentation and add an example to make clear this isn't about
generic masks in registers. Also, fix the doc regarding read operations:
the mask part has no effect on them.

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20231205155820.2133813-1-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>