[PATCH][GAS] aarch64: Update AArch64 features command line options docs 1/2

[Przemyslaw Wirkus]

> Hi Przemyslaw,
> 
> > 2021-09-20  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>
> >
> > 	* doc/c-aarch64.texi: Update docs.
> 
> Approved - please apply.

commit 7645513a26a88f118743f06f5980c8913b550eb8

> Cheers
>    Nick
>