[GCC, DOCS, AArch64] Add missing documenation for mbranch-protection

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

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.
+
 2019-04-03  Richard Biener  <rguenther@suse.de>
 
 	PR lto/89896

gcc/doc/extend.texi

@@ -3925,7 +3925,15 @@ same as for the @option{-mcpu=} command-line option.
 @cindex @code{sign-return-address} function attribute, AArch64
 Select the function scope on which return address signing will be applied.  The
 behavior and permissible arguments are the same as for the command-line option
-@option{-msign-return-address=}.  The default value is @code{none}.
+@option{-msign-return-address=}.  The default value is @code{none}.  This
+attribute is deprecated.  The @code{branch-protection} attribute should
+be used instead.
+
+@item branch-protection
+@cindex @code{branch-protection} function attribute, AArch64
+Select the function scope on which branch protection will be applied.  The
+behavior and permissible arguments are the same as for the command-line option
+@option{-mbranch-protection=}.  The default value is @code{none}.
 
 @end table
 

gcc/doc/invoke.texi

@@ -632,7 +632,7 @@ Objective-C and Objective-C++ Dialects}.
 -mlow-precision-recip-sqrt  -mlow-precision-sqrt  -mlow-precision-div @gol
 -mpc-relative-literal-loads @gol
 -msign-return-address=@var{scope} @gol
--mbranch-protection=@var{none}|@var{standard}|@var{pac-ret}[+@var{leaf}] @gol
+-mbranch-protection=@var{none}|@var{standard}|@var{pac-ret}[+@var{leaf}]|@var{bti} @gol
 -march=@var{name}  -mcpu=@var{name}  -mtune=@var{name}  @gol
 -moverride=@var{string}  -mverbose-cost-dump @gol
 -mstack-protector-guard=@var{guard} -mstack-protector-guard-reg=@var{sysreg} @gol
@@ -15878,7 +15878,7 @@ functions, and @samp{all}, which enables pointer signing for all functions.  The
 default value is @samp{none}. This option has been deprecated by
 -mbranch-protection.
 
-@item -mbranch-protection=@var{none}|@var{standard}|@var{pac-ret}[+@var{leaf}]
+@item -mbranch-protection=@var{none}|@var{standard}|@var{pac-ret}[+@var{leaf}]|@var{bti}
 @opindex mbranch-protection
 Select the branch protection features to use.
 @samp{none} is the default and turns off all types of branch protection.