libgomp.texi: Update 'Device Information Routines' section

Update 'OpenMP Runtime Library Routines' by adding a note that invoking
inside a target region might invoke unspecified behavior. Additionally,
update omp_{get,set}_default_device for omp_{initial,invalid}_device
named constants.

libgomp/ChangeLog:

	* libgomp.texi (OpenMP Runtime Library Routines): Add missing
	title to some commented still undocumented items.
	(Device Information Routines): Update.