bpf: fix documentation for eBPF helpers

Minor formatting edits for eBPF helpers documentation, including blank
lines removal, fix of item list for return values in bpf_fib_lookup(),
and missing prefix on bpf_skb_load_bytes_relative().

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