Attached is a documentation-only patch to clarify the use case
of multiple distinct format_arg attributes on a single function.
It's far from obvious that the use case makes sense so explicitly
mentioning it should help avoid the mistake of assuming that
accepting it is due to the lack of better checking in the area
(it could also prompt Clang to support the use case -- likely
an omission due to its obscurity).

However, since it makes less sense to declare the same function
multiple times with distinct format_arg attributes, a follow-on
improvement would be to issue a warning on such instances.  I'll
leave the bug open until that's implemented.

Martin