bpf/docs: Document cpumask kfuncs in a new file

Now that we've added a series of new cpumask kfuncs, we should document
them so users can easily use them. This patch adds a new cpumasks.rst
file to document them.
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230125143816.721952-6-void@manifault.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/index.rst

@@ -20,6 +20,7 @@ that goes into great technical depth about the BPF Architecture.
    syscall_api
    helpers
    kfuncs
+   cpumasks
    programs
    maps
    bpf_prog_run

Documentation/bpf/kfuncs.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. _kfuncs-header-label:
+
 =============================
 BPF Kernel Functions (kfuncs)
 =============================
@@ -420,3 +424,10 @@ the verifier. bpf_cgroup_ancestor() can be used as follows:
 		bpf_cgroup_release(parent);
 		return 0;
 	}
+
+3.3 struct cpumask * kfuncs
+---------------------------
+
+BPF provides a set of kfuncs that can be used to query, allocate, mutate, and
+destroy struct cpumask * objects. Please refer to :ref:`cpumasks-header-label`
+for more details.