Documentation: clarify firmware_class provenance and why we can't rename the module

Clarify the provenance of the firmware loader firmware_class module name
and why we cannot rename the module in the future.

Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>