x86/mtrr: Document missing function parameters in kernel-doc

Add text explaining what they do.

No functional changes.

Closes: https://lore.kernel.org/oe-kbuild-all/202311130104.9xKAKzke-lkp@intel.com/


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/202311130104.9xKAKzke-lkp@intel.com