Document strftime %Ob and %OB as C2X features.

C2X standardizes strftime %Ob and %OB support.  This patch updates the
glibc manual to say these are C2X features, while noting that the
details of what is the alternative form of a month name are not
specified in C2X.

Note: C2X seems unclear to me about whether %h being equivalent to %b
means %Oh is thereby allowed and equivalent to %Ob; I've asked WG14.

Tested with "make info" and "make pdf".

	* manual/time.texi (strftime): Document %Ob and %OB as C2X
	features.