Clarify trigger command help.

Author: hjoliver

cylc/flow/scripts/trigger.py

@@ -27,7 +27,7 @@
 running) at once. They release tasks to run when their active task count
 drops below the queue limit.
 
-Attempts to trigger active tasks will be ignored.
+Attempts to trigger active (submitted, running) tasks will be ignored.
 
 Examples:
   # trigger task foo in cycle 1234 in test
@@ -40,7 +40,7 @@
   $ cylc trigger --flow=new test//1234/foo
 
 Flows:
-  Active tasks (in the n=0 window) already belong to a flow.
+  Waiting tasks in the active window (n=0) already belong to a flow.
   * by default, if triggered, they run in the same flow
   * or with --flow=all, they are assigned all active flows
   * or with --flow=INT or --flow=new, the original and new flows are merged