mesa: Document that _mesa_enum_to_string() returns non-null (v2)

It always returns non-null, even if the number is an invalid enum.

Cc: Haixia Shi <hshi@chromium.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Change-Id: I26e8843c96130be972e66f48a49e362442e1bf97
1 file changed