chore(config): document options for audit logs (#916)

Adds documentation for the auditEnabled and auditFile config options in
syncthing/syncthing#10066.

Author: marbens-arch

users/config.rst

@@ -1349,6 +1349,21 @@ The ``options`` element contains all other global configuration options.
     connections.  The mechanism is described in detail in a :doc:`separate
     chapter </advanced/option-connection-limits>`.
 
+.. option:: options.auditEnabled
+
+    When ``true``, analogous to :option:`--audit` being set.
+    Defaults to ``false``.
+
+    When either this option, or :option:`--audit` (or both) are enabled,
+    auditing is enabled.
+
+.. option:: options.auditFile
+
+    Analogous to :option:`--auditfile`. Defaults to being unset.
+
+    For compatibility reasons, if both this option and :option:`--auditfile`
+    are set, :option:`--auditfile` takes priority.
+
 Defaults Element
 ----------------