Add clarification advised by @TallJimbo

doc/lsst.pipe.base/creating-a-pipelinetask.rst

@@ -475,11 +475,27 @@ subtraction.
 
 To bring this all together, see :ref:`pipeline-appendix-c`.
 
-Note that the same mechanism works for output connections:
+The same mechanism works for output connections:
 an output from a `PipelineTask` can be made optional (under config control)
 or a config switch can even be used to choose between different ways of
 providing output.
 
+Note that disabling a connection via this mechanism, during initialization,
+has the same effect on quantum graph generation and execution as if the
+connection had never existed.
+
+Run-time optional inputs
+------------------------
+
+A separate mechanism exists that allows an `Input` connection to be made
+run-time optional.
+If the `Input.minimum` attribute is initialized to zero for a connection,
+graph-building will still generate a quantum, and the `PipelineTask` will
+be run, even if no dataset for that input can be found.
+
+Output connections are run-time optional by definition; it is always possible
+for a `PipelineTask` to produce no output; that by itself is not treated as
+an error condition.
 
 ----------------------------------------
 Dataset name configuration and templates