[arm][doc] Document accepted -march=armv8.3-a extension options

I noticed that we helpfully list the extensions that are accepted
by the -march options on arm but we were missing the information
for 'armv8.3-a'.

This patchlet corrects that.
Built the documentation and it looked ok.

	* doc/invoke.texi (ARM Options): Document accepted extension options
	for -march=armv8.3-a.

From-SVN: r255859

gcc/ChangeLog

+2017-12-20  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
+
+	* doc/invoke.texi (ARM Options): Document accepted extension options
+	for -march=armv8.3-a.
+
 2017-12-20  Richard Earnshaw  <rearnsha@arm.com>
 
 	PR target/83105

gcc/doc/invoke.texi

@@ -15853,6 +15853,7 @@ Disable the floating-point, Advanced SIMD and cryptographic instructions.
 @end table
 
 @item armv8.2-a
+@itemx armv8.3-a
 @table @samp
 @item +fp16
 The half-precision floating-point data processing instructions.