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.