fw_cfg: document fw_cfg_modify_iXX() update functions

Document the behavior of fw_cfg_modify_iXX() for leak-less updating
of integer-type blobs.

Currently only fw_cfg_modify_i16() is coded, but 32- and 64-bit versions
may be added later if necessary..

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
