landlock: Clarify documentation for struct landlock_ruleset_attr

The explanation for @handled_access_fs and @handled_access_net has
significant overlap and is better explained together.

* Explain the commonalities in structure-level documentation.
* Clarify some wording and break up longer sentences.
* Put emphasis on the word "handled" to make it clearer that "handled"
  is a term with special meaning in the context of Landlock.

I'd like to transfer this wording into the man pages as well.

Signed-off-by: Günther Noack <gnoack@google.com>
Cc: Alejandro Colomar <alx@kernel.org>
Cc: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Cc: linux-security-module@vger.kernel.org
Link: https://lore.kernel.org/r/20240711165456.2148590-2-gnoack@google.com


[mic: Format commit message]
Signed-off-by: Mickaël Salaün <mic@digikod.net>