OpenMP: Update documentation of metadirective implementation status.

libgomp/ChangeLog
	* libgomp.texi (OpenMP 5.0): Mark metadirective and declare variant
	as implemented.
	(OpenMP 5.1): Mark target_device as supported.
	Add changed interaction between declare target and OpenMP context
	and dynamic selector support.
	(OpenMP 5.2): Mark otherwise clause as supported, note that
	default is also still accepted.