docs: add reference to --wait-for

docs/how-to-guides/snap-development/use-snapctl.md

@@ -85,7 +85,9 @@ $ snapctl get --view :setup-wifi routes --with to=default
 
 Concurrent accesses may block if they would result in writes running concurrently with another access. If any access is blocked, subsequent accesses will also be queued to ensure that they are run in order.
 
-For example, if there is a read ongoing and a write is requested, the write will be blocked. A new read would then also be queued and only run after the write, even though reads are normally run concurrently. This ensures that a stream of reads cannot starve a write access.
+For example, if there is an ongoing read and a write is requested, the write will be blocked. A new read would then also be queued and only run after the write finishes, even though reads are normally run concurrently. This ensures that a stream of reads cannot starve a write access.
+
+The `--wait-for` flag can be used to specify a timeout after which the command will stop waiting for other accesses to complete and cancel the request.
 
 For further information on confdb, see {ref}`Configure snaps with confdb <how-to-guides-manage-snaps-configure-snaps-with-confdb>` and {ref}`Confdb configuration mechanism <explanation-how-snaps-work-confdb-configuration-mechanism>`.