[PATCH, gas/doc] Document .xword Aarch64 directive

[Nick Clifton]

Hi Thomas,

> *** gas/ChangeLog ***
>
> 2015-07-10  Thomas Preud'homme  <thomas.preudhomme@arm.com>
>
>          * doc/c-aarch64.texi (.xword): Document directive.
>

Approved - please apply.

Cheers
   Nick