Add docs for Timer class (#903)

Another class that has remained undocumented for a long time.

doc/site/cli/modules/timer/timer.markdown

@@ -1,7 +1,12 @@
 ^title Timer Class
 
-**TODO**
+## Static Method
 
-## Methods
+### Timer.**sleep**(milliseconds)
+
+Suspends the current fiber for the given number of milliseconds. It is a runtime error if this is not a non-negative number.
+
+This method is often used in conjunction with the Scheduler class which runs any scheduled tasks in separate fibers whilst the current fiber is sleeping.
+
+Note that this method also suspends the System.clock method which will not give the correct running time for a program as a result.
 
-**TODO**