bpf: fix documentation for eBPF helpers

Another round of minor fixes for the documentation of the BPF helpers
located in the UAPI bpf.h header file. Changes include:

- Moving around description of some helpers, to keep the descriptions in
  the same order as helpers are declared (bpf_map_push_elem(), leftover
  from commit 90b1023f ("bpf: fix documentation for eBPF helpers"),
  bpf_rc_keydown(), and bpf_skb_ancestor_cgroup_id()).
- Fixing typos ("contex" -> "context").
- Harmonising return types ("void* " -> "void *", "uint64_t" -> "u64").
- Addition of the "bpf_" prefix to bpf_get_storage().
- Light additions of RST markup on some keywords.
- Empty line deletion between description and return value for
  bpf_tcp_sock().
- Edit for the description for bpf_skb_ecn_set_ce() (capital letters,
  acronym expansion, no effect if ECT not set, more details on return
  value).

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>