libgomp.texi: Document omp(x)::allocator::*, restructure memory allocator doc

libgomp.texi: Document omp(x)::allocator::*, restructure memory allocator doc

libgomp/ChangeLog:

	* libgomp.texi (omp_init_allocator): Refer to 'Memory allocation'
	for available memory spaces.
	(OMP_ALLOCATOR): Move list of traits and predefined memspaces
	and allocators to ...
	(Memory allocation): ... here. Document omp(x)::allocator::*;
	minor wording tweaks, be more explicit about memkind, pinned and
	pool_size.

Co-authored-by: waffl3x <waffl3x@baylibre.com>