Documentation/bpf: Add documentation for filesystem kfuncs

Add a brief introduction for file system kfuncs:

  bpf_get_file_xattr()
  bpf_get_fsverity_digest()

The documentation highlights the strategy to avoid recursions of these
kfuncs.

Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231129234417.856536-4-song@kernel.org


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