Due to the nature of templates, it is not immediately obvious what a template does, especially with extremely complex templates, or templates made of other templates. Comments are particularly helpful in situations like these.
Comments can be used to help explain complex templates so that you remember what they are used for and others can work on the project more effectively.
Comments can be typed out in the comment box in the property tab of the node selected. Comments can be added to undocked nodes or the top node only, of a docked node.
For instance, to add a comment to a template within a namespace you just click the namespace node and make a comment in the properties panel for that template. Comments will be unique to their node instances.
Comment options are collapsed by default in the properties panel. Uncollapse the comment section to add a comment.
As soon as you start typing a comment, three more properties will appear.
Scale: The size of the comment text.
Auto Wrap: whether to wrap the text automatically or let you manually decide the line breaks.
Wrap Length: The length of each comment line.