bpf: add documentation for eBPF helpers (33-41)

Add documentation for eBPF helper functions to bpf.h user header file.
This documentation can be parsed with the Python script provided in
another commit of the patch series, in order to provide a RST document
that can later be converted into a man page.

The objective is to make the documentation easily understandable and
accessible to all eBPF developers, including beginners.

This patch contains descriptions for the following helper functions, all
written by Daniel:

- bpf_get_hash_recalc()
- bpf_skb_change_tail()
- bpf_skb_pull_data()
- bpf_csum_update()
- bpf_set_hash_invalid()
- bpf_get_numa_node_id()
- bpf_set_hash()
- bpf_skb_adjust_room()
- bpf_xdp_adjust_meta()

v4:
- bpf_skb_change_tail(): Clarify comment about invalidated verifier
  checks.
- bpf_skb_pull_data(): Clarify the motivation for using this helper or
  bpf_skb_load_bytes(), on non-linear buffers. Fix RST formatting for
  *skb*. Clarify comment about invalidated verifier checks.
- bpf_csum_update(): Fix description of checksum (entire packet, not IP
  checksum). Fix a typo: "header" instead of "helper".
- bpf_set_hash_invalid(): Mention bpf_get_hash_recalc().
- bpf_get_numa_node_id(): State that the helper is not restricted to
  programs attached to sockets.
- bpf_skb_adjust_room(): Clarify comment about invalidated verifier
  checks.
- bpf_xdp_adjust_meta(): Clarify comment about invalidated verifier
  checks.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>