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

[Nick Clifton]

Hi Przemyslaw,

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

Approved - please apply.

Cheers
   Nick