bpf, docs: Document existing macros instead of deprecated

As mentioned above, a nested pointer obtained from walking a trusted pointer is

no longer trusted, with one exception. If a struct type has a field that is

guaranteed to be valid as long as its parent pointer is trusted, the

``BTF_TYPE_SAFE_NESTED`` macro can be used to express that to the verifier as

follows:

guaranteed to be valid (trusted or rcu, as in KF_RCU description below) as long

as its parent pointer is valid, the following macros can be used to express

that to the verifier:

* ``BTF_TYPE_SAFE_TRUSTED``

* ``BTF_TYPE_SAFE_RCU``

* ``BTF_TYPE_SAFE_RCU_OR_NULL``

For example,

.. code-block:: c

	BTF_TYPE_SAFE_TRUSTED(struct socket) {

		struct sock *sk;

	};

or

.. code-block:: c

	BTF_TYPE_SAFE_NESTED(struct task_struct) {

	BTF_TYPE_SAFE_RCU(struct task_struct) {

		const cpumask_t *cpus_ptr;

		struct css_set __rcu *cgroups;

		struct task_struct __rcu *real_parent;

		struct task_struct *group_leader;

	};

In other words, you must:

1. Wrap the trusted pointer type in the ``BTF_TYPE_SAFE_NESTED`` macro.

1. Wrap the valid pointer type in a ``BTF_TYPE_SAFE_*`` macro.

2. Specify the type and name of the trusted nested field. This field must match

2. Specify the type and name of the valid nested field. This field must match

   the field in the original type definition exactly.

A new type declared by a ``BTF_TYPE_SAFE_*`` macro also needs to be emitted so

that it appears in BTF. For example, ``BTF_TYPE_SAFE_TRUSTED(struct socket)``

is emitted in the ``type_is_trusted()`` function as follows:

.. code-block:: c

	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct socket));

2.4.5 KF_SLEEPABLE flag

-----------------------