net: clarify SO_DEVMEM_DONTNEED behavior in documentation

Document new behavior when the number of frags passed is too big.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20241107210331.3044434-2-almasrymina@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>