manual: Document unsupported cases for interposition

These functions call the core allocator functions (realloc and malloc
respectively) and are hence guaranteed to allocate memory using the
correct functions when multiple allocators are interposed.  Having
these functions interposed in one allocator and not another may result
in confusion, hence discourage interposing them altogether.

Reviewed-by: Florian Weimer <fweimer@redhat.com>