landlock: Various documentation improvements

landlock: Various documentation improvements

* Fix some typos, incomplete or confusing phrases.
* Split paragraphs where appropriate.
* List the same error code multiple times,
  if it has multiple possible causes.
* Bring wording closer to the man page wording,
  which has undergone more thorough review
  (esp. for LANDLOCK_ACCESS_FS_WRITE_FILE).
* Small semantic clarifications
  * Call the ephemeral port range "ephemeral"
  * Clarify reasons for EFAULT in landlock_add_rule()
  * Clarify @rule_type doc for landlock_add_rule()

This is a collection of small fixes which I collected when preparing the
corresponding man pages [1].

Cc: Alejandro Colomar <alx@kernel.org>
Cc: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20240715155554.2791018-1-gnoack@google.com
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20240715160328.2792835-2-gnoack@google.com
[mic: Add label to link, fix formatting spotted by make htmldocs,
synchronize userspace-api documentation's date]
Signed-off-by: Mickaël Salaün <mic@digikod.net>