scripts: kernel-doc: support private / public marking for enums

Enums benefit from private markings, too. For netlink attribute
name enums always end with a pair of __$n_MAX and $n_MAX members.
Documenting them feels a bit tedious.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20230621223525.2722703-1-kuba@kernel.org>