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

src/mapi/glapi/gen/gl_enums.py

@@ -83,7 +83,12 @@ static int compar_nr( const int *a, enum_elt *b )
 
 static char token_tmp[20];
 
-const char *_mesa_enum_to_string( int nr )
+/**
+ * This function always returns a string. If the number is a valid enum, it
+ * returns the enum name. Otherwise, it returns a numeric string.
+ */
+const char *
+_mesa_enum_to_string(int nr)
 {
    enum_elt *elt;