proc_sysctl: update docs for __register_sysctl_table()

Update the docs for __register_sysctl_table() to make it clear no child
entries can be passed. When the child is true these are non-leaf entries
on the ctl table and sysctl treats these as directories. The point to
__register_sysctl_table() is to deal only with directories not part of
the ctl table where thay may riside, to be simple and avoid recursion.

While at it, hint towards using long on extra1 and extra2 later.

Cc: stable@vger.kernel.org # v5.17
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>