Clarifies introduction.

Also adds a stub, following advice in #3209. Adds a missing method stub, refs #3321.

Author: JJ

doc/Type/Scheduler.pod6

@@ -8,8 +8,9 @@
         has &.uncaught_handler is rw
     }
 
-Common role for schedulers. A scheduler is a piece of code that determines
-which resources to use to run which task, and when.
+A scheduler is a piece of code that determines which resources to use to run
+which task, and when. This role contains code that will be the same for all
+kinds of schedulers.
 
 Some operations for example on L<Proc::Async|/type/Proc::Async>,
 L<Promise|/type/Promise>, L<Supply|/type/Supply> allow you to specify a
@@ -63,6 +64,14 @@ will be thrown. This only applies to releases 2019.05 and later.
 One should call the C<cancel> method on the returned C<Cancellation> object
 to cancel the (possibly repeated) cueing of the code.
 
+=head2 method loads
+
+Defined as:
+
+    method loads()
+
+This is a method stub, should be re-implemented when subclassing.
+
 =end pod
 
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