bpf: Clarify BPF_RB_NO_WAKEUP behavior for bpf_ringbuf_discard()

Clarify bpf_ringbuf_discard() documentation for BPF_RB_NO_WAKEUP.

Discarded ring buffer records are still left in the ring buffer and are
only skipped when user space consumes them. This can matter when
BPF_RB_NO_WAKEUP is used: a later submit relying on adaptive wakeup
might not wake the consumer, because the discarded record still needs to
be consumed first.

Scenario:

epoll_wait(rb_fd); // blocks

rec = bpf_ringbuf_reserve(&rb, ...);
bpf_ringbuf_discard(rec, BPF_RB_NO_WAKEUP);

rec = bpf_ringbuf_reserve(&rb, ...);
bpf_ringbuf_submit(rec, 0); // valid record, but no wakeup

Document this in bpf_ringbuf_discard() to make the interaction between
discarded records, user-space consumption, and adaptive wakeups explicit.

Reported-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260331130612.3762433-1-eyal.birger@gmail.com

----

v2: adapt wording per feedback from Andrii.