The BTF_TYPE_SAFE_NESTED macro was replaced by the BTF_TYPE_SAFE_TRUSTED,
BTF_TYPE_SAFE_RCU, and BTF_TYPE_SAFE_RCU_OR_NULL macros. Fix the docs
correspondingly.
Fixes: 6fcd486b3a0a ("bpf: Refactor RCU enforcement in the verifier.")
Signed-off-by: Anton Protopopov <aspsk@xxxxxxxxxxxxx>