[PATCH 15/15][ARM] Update sourcebuild.texi with testsuite/effective-target hooks

[Alan Lawrence <alan dot lawrence at arm dot com>]

This documents the change to arm_neon_fp16_ok in the first patch; the addition 
of arm_neon_fp16_hw_ok in the last patch; and corrects a cross-reference.

(I tried using an @ref instead of "Implies previous." but the page ref looked 
very out-of-place in PDF when I am referring to the previous item in the list!)

gcc/ChangeLog:

	* doc/sourcebuild.texi (arm_neon_fp16): Correct cross-reference.
	(arm_neon_fp16_ok): Document adding of -mfp16-format=ieee flag.
	(arm_neon_fp16_hw_ok): New.

diff --git a/gcc/doc/sourcebuild.texi b/gcc/doc/sourcebuild.texi
index 13f63d1..0c0fe84 100644
--- a/gcc/doc/sourcebuild.texi
+++ b/gcc/doc/sourcebuild.texi
@@ -1549,7 +1549,12 @@ options.  Some multilibs may be incompatible with these options.
 @item arm_neon_fp16_ok
 @anchor{arm_neon_fp16_ok}
 ARM Target supports @code{-mfpu=neon-fp16 -mfloat-abi=softfp} or compatible
-options.  Some multilibs may be incompatible with these options.
+options, including @code{-mfp16-format=ieee} if necessary to obtain the
+@code{__fp16} type.  Some multilibs may be incompatible with these options.
+
+@item arm_neon_fp16_hw_ok
+Test system supports executing Neon half-precision float instructions.
+(Implies previous.)
 
 @item arm_thumb1_ok
 ARM target generates Thumb-1 code for @code{-mthumb}.
@@ -2016,7 +2021,7 @@ keyword}.
 @item arm_neon_fp16
 NEON and half-precision floating point support.  Only ARM targets
 support this feature, and only then in certain modes; see
-the @ref{arm_neon_ok,,arm_neon_fp16_ok effective target keyword}.
+the @ref{arm_neon_fp16_ok,,arm_neon_fp16_ok effective target keyword}.
 
 @item arm_vfp3
 arm vfp3 floating point support; see