bpf: add documentation for eBPF helpers (12-22)

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 Alexei:

- bpf_get_current_pid_tgid()
- bpf_get_current_uid_gid()
- bpf_get_current_comm()
- bpf_skb_vlan_push()
- bpf_skb_vlan_pop()
- bpf_skb_get_tunnel_key()
- bpf_skb_set_tunnel_key()
- bpf_redirect()
- bpf_perf_event_output()
- bpf_get_stackid()
- bpf_get_current_task()

v4:
- bpf_redirect(): Fix typo: "XDP_ABORT" changed to "XDP_ABORTED". Add
  note on bpf_redirect_map() providing better performance. Replace "Save
  for" with "Except for".
- bpf_skb_vlan_push(): Clarify comment about invalidated verifier
  checks.
- bpf_skb_vlan_pop(): Clarify comment about invalidated verifier
  checks.
- bpf_skb_get_tunnel_key(): Add notes on tunnel_id, "collect metadata"
  mode, and example tunneling protocols with which it can be used.
- bpf_skb_set_tunnel_key(): Add a reference to the description of
  bpf_skb_get_tunnel_key().
- bpf_perf_event_output(): Specify that, and for what purpose, the
  helper can be used with programs attached to TC and XDP.

v3:
- bpf_skb_get_tunnel_key(): Change and improve description and example.
- bpf_redirect(): Improve description of BPF_F_INGRESS flag.
- bpf_perf_event_output(): Fix first sentence of description. Delete
  wrong statement on context being evaluated as a struct pt_reg. Remove
  the long yet incomplete example.
- bpf_get_stackid(): Add a note about PERF_MAX_STACK_DEPTH being
  configurable.

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