AArch64, ARM: Clean up documentation of -mbranch-protection.

AArch64, ARM: Clean up documentation of -mbranch-protection.

While working on other things, I noticed that the documentation for
the -mbranch-protection= option was pretty garbled on both aarch64 and
arm targets, with incorrect markup, too much syntax crammed into the
option summary, and confusion about what the values "+leaf" modifier
can apply to.  I rewrote it to list all the valid option values
explicitly in the option description, checking this against the
implementation.

gcc/ChangeLog
	* doc/invoke.texi (AArch64 Options): Clean up description of
	-mbranch-protection= argument.
	(ARM Options): Likewise.