landlock: Clarify IPC scoping documentation

* Clarify terminology
* Stop mixing the unix(7) and signal(7) aspects in the explanation.

Terminology:

* The *IPC Scope* of a Landlock domain is that Landlock domain and its
  nested domains.
* An *operation* (e.g., signaling, connecting to abstract UDS) is said to
  be *scoped within a domain* when the flag for that operation was set at
  ruleset creation time.  This means that for the purpose of this
  operation, only processes within the domain's IPC scope are reachable.

Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250303194510.135506-4-gnoack@google.com


[mic: Update doc date]
Signed-off-by: Mickaël Salaün <mic@digikod.net>