docs: Use @var{S} etc. in Spec File invoke.texi documentation

docs: Use @var{S} etc. in Spec File invoke.texi documentation

We got internally a question about the Spec File syntax, misunderstanding
what is the literal syntax and what are the placeholder variables in
the syntax descriptions.
The following patch attempts to use @var{S} etc. instead of just S
to clarify it stands for any option (or start of option etc.) rather
than literal S, say in %{S:X}.  At least in HTML documentation it
then uses italics.

2024-03-27  Jakub Jelinek  <jakub@redhat.com>

	* doc/invoke.texi (Spec Files): Use @var{S} instead of S,
	@var{X} instead of X etc. for other placeholders.