[Ada] Clarify hardening command-line options that require explicit choices

Prefixes -fzero-call-used-regs and -fstrub could be mistaken for full
command-line options with the references to them in the GNAT RM.  Make
it clearer that they require explicit choices.

gcc/ada/

	* doc/gnat_rm/security_hardening_features.rst: Clarify the need
	for choices after -fzero-call-used-regs and -fstrub.
	* gnat_rm.texi: Regenerate.