Update manual references for C17 having been published.

Update manual references for C17 having been published.

This patch makes references in the manual to C17 reflect it having
been published in July 2018.  (For the reasons it took so long to get
to ballot and publication, see the WG14 convenor's report to the last
SC22 plenary - SC22 N5297 - where it references "A troubling new trend
where ISO CS has begun enforcing undocumented rules that are approved
neither by the TMB nor by the WG 14 editors, leading to a decrease in
the usefulness of standards documents.".)

Tested with "make info html pdf".

	* doc/invoke.texi (-std=c17), doc/standards.texi (C Language):
	Document C17 as published in 2018.

From-SVN: r265249