mm: vrealloc: properly document __GFP_ZERO behavior

mm: vrealloc: properly document __GFP_ZERO behavior

Properly document that if __GFP_ZERO logic is requested, callers must
ensure that, starting with the initial memory allocation, every subsequent
call to this API for the same memory allocation is flagged with
__GFP_ZERO.  Otherwise, it is possible that __GFP_ZERO is not fully
honored by this API.

Link: https://lkml.kernel.org/r/20240730185049.6244-4-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>