Doc: Document enum with underlying type extension [PR117689]

Doc: Document enum with underlying type extension [PR117689]

This is a C23/C++11 feature that is supported as an extension with
earlier -std= options too, but was never previously documented.  It
interacts with the already-documented forward enum definition extension,
so I have merged discussion of the two extensions into the same section.

gcc/ChangeLog
	PR c/117689
	* doc/extend.texi (Incomplete Enums): Rename to....
	(Enum Extensions): This.  Document support for specifying the
	underlying type of an enum as an extension in all earlier C
	and C++ standards.  Document that a forward declaration with
	underlying type is not an incomplete type, and which dialects
	GCC supports that in.