bpf: fix minor issues in documentation for BPF helpers.

This commit brings many minor fixes to the documentation for BPF helper
functions. Mostly, this is limited to formatting fixes and improvements.
In particular, fix broken formatting for bpf_skb_adjust_room().

Besides formatting, replace the mention of "bpf_fullsock()" (that is not
associated with any function or type exposed to the user) in the
description of bpf_sk_storage_get() by "full socket".

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>