docs: Add 'S' to Machine Constraints for RISC-V

docs: Add 'S' to Machine Constraints for RISC-V

It was undocument before, but it might used in linux kernel for resolve
code model issue, so LLVM community suggest we should document that,
so that make it become supported/documented/non-internal machine constraints.

gcc/ChangeLog:

	PR target/101275
	* config/riscv/constraints.md ("S"): Update description and remove
	@internal.
	* doc/md.texi (Machine Constraints): Document the 'S' constraints
	for RISC-V.