Cluster legend: use descriptive text instead of diagram

[ajeanmahoney]

Use the following text description as the legend instead of the diagram.

• Blue - doc is In Progress and can move on its own through the queue (i.e., does not have a normative reference to an I-D)
• Green - doc is In Progress because its normative reference is in the queue
• Pink - doc is blocked because its normative reference is not in the queue (a state previously known as MISSREF)
• Orange - a referenced doc that is not in the queue. This doc needs to be in the queue for blocked docs in the cluster to move.

I've received user feedback that the above text is easier to understand than the diagram.

h/t @evyncke @debcooley

[evyncke]

Also some text about the direction of the arrow ;-)