This patch add the missing documentation bits for -mbranch-protection in both
extend.texi and invoke.texi.

*** gcc/ChangeLog ***

2019-04-03  Sudakshina Das  <sudi.das@arm.com>

	* doc/extend.texi: Add deprecated comment on sign-return-address
	function attribute and add mbranch-protection.
	* doc/invoke.texi: Add bti to the options for mbranch-protection.

From-SVN: r270119