Are you aware of the recently introduced indicator when a node includes a description?
This button takes you to the description edit view:
You can include Markup and even mermaid diagrams.
Click on the Expand button (circled red above) and then the Preview button to display the description nicely formatted:
A node which has such a description gets an indicator in the editor (this is the new bit) :
.
Clicking the little badge takes you to the description input for the node.
While viewing the description takes more clicks than ideal, you can certainly use it to fully document your flow and individual nodes.
My example uses @gregorius' https://discourse.nodered.org/t/node-red-flow-to-mermaid-diagram/81412 since he explains how to include a flow diagram.
Note that a flow tab has the same Description editor, but there is no badge shown on the tab to indicate that it is in use.
