bpf,docs: Remove bpf_cpumask_kptr_get() from documentation

Now that the kfunc no longer exists, we can remove it and instead
describe how RCU can be used to get a struct bpf_cpumask from a map
value. This patch updates the BPF documentation accordingly.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-6-void@manifault.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>