Toggle `buffer-face-mode', using face specs SPECS.
Each argument in SPECS should be a face, i.e. either a face name
or a property list of face attributes and values. If more than
one face is listed, that specifies an aggregate face, like in a
`face' text property.
If `buffer-face-mode' is already enabled, and is currently using
the face specs SPECS, then it is disabled; if buffer-face-mode is
disabled, or is enabled and currently displaying some other face,
then is left enabled, but the face changed to reflect SPECS.
This function will make the variable `buffer-face-mode-face'
buffer local, and set it to SPECS. (fn &rest SPECS)