bpf, doc: clarification for the meaning of 'id'

For me, as a reader whose mother language isn't English, the
old words bring a little difficulty to catch the meaning, this
patch rewords the subsection in a more clarificatory way.

This patch also add blank lines as separator at two places
to improve readability.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>