bpf: Fix helpers ctx struct types in uapi doc

Helpers may operate on two types of ctx structures: user visible ones
(e.g. `struct bpf_sock_ops`) when used in user programs, and kernel ones
(e.g. `struct bpf_sock_ops_kern`) in kernel implementation.

UAPI documentation must refer to only user visible structures.

The patch replaces references to `_kern` structures in BPF helpers
description by corresponding user visible structures.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>