arm: doc: Update documentation on half-precision support

arm: doc: Update documentation on half-precision support

The previous patch makes __fp16 always available on arm (using
-mfp16-format is no longer needed), so the documentation needs an
update.

In the process, clarify the peculiarities of __fp16 on arm, and
reorder information to make it easier to understand.

gcc/ChangeLog:
	* doc/extend.texi (Half-precision Floating-point): __fp16 is now
	always available on arm.  Move x86 paragraph closer to the rest of
	the x86 information, and make it use present tense.