Add preamble for Scheduler module (#919)

We need some opening remarks here to get rid of the TODO.

doc/site/cli/modules/scheduler/index.markdown

@@ -1,5 +1,7 @@
 ^title Module "scheduler"
 
-**TODO**
+This module provides a vehicle to allow other operations to be performed asynchronously whilst waiting for the main operation to be completed.
+
+It contains a single class:
 
 * [Scheduler](scheduler.html)