Follow up to: https://gcc.gnu.org/ml/gcc-patches/2020-02/msg01109.html

Committed as obvious.

gcc/ChangeLog:

2020-02-21  Mihail Ionescu  <mihail.ionescu@arm.com>

	* doc/sourcebuild.texi (arm_v8_1m_mve_ok):
	Document new target supports option.