overflow: Clarify expectations for getting DEFINE_FLEX variable sizes

Mention the use of __member_size() for DEFINE_FLEX variables as a hint
for getting at the compile-time size of the resulting flexible array
member.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20250416172911.work.854-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>