doc: Improve documentation of -Wchar-subscripts and char signedness [PR94182]

The documentation of -Wchar-subscripts was missing cross-references to
discussion elsewhere in the manual about how the char type may be either
signed or unsigned.  Besides adding the references, I've also done some
improvements to those two other sections.

gcc/ChangeLog
	PR other/94182
	* doc/implement-c.texi (Characters implementation): Add
	cross-reference to more detailed discussion about signedness.
	* doc/invoke.texi (C Dialect Options): Combine -fsigned-char
	and -funsigned-char table entries since the following discussion
	applies to both.  Add an anchor so other things can point at it.
	(Warning Options): Add cross-references to the above two places.
	Clarify that the warning is given regardless of whether char is
	signed or unsigned.