drm/i915/pxp: add PXP documentation

Now that all the pieces are in place we can add a description of how the
feature works. Also modify the comments in struct intel_pxp into
kerneldoc.

v2: improve doc (Rodrigo)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@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/20210924191452.1539378-17-alan.previn.teres.alexis@intel.com