Improve doxygen comments for allocators in containers

Improve doxygen comments for allocators in containers

	PR libstdc++/70716
	* include/bits/forward_list.h (forward_list): Update doxygen comments
	to reflect allocator propagation semantics. Remove ambiguous
	statements about data being lost.
	* include/bits/stl_deque.h (deque): Likewise.
	* include/bits/stl_list.h (list): Likewise.
	* include/bits/stl_map.h (map): Likewise.
	* include/bits/stl_multimap.h (multimap): Likewise.
	* include/bits/stl_multiset.h (multiset): Likewise.
	* include/bits/stl_set.h (set): Likewise.
	* include/bits/stl_vector.h (vector): Likewise.
	* include/bits/unordered_map.h (unordered_map, unordered_multimap):
	Likewise.
	* include/bits/unordered_set.h (unordered_set, unordered_multiset):
	Likewise.

From-SVN: r238332