core node doc: The Comment node

src/_includes/core-nodes/comment-use-case.md

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+## What's the Comment node in Node-RED used for?
+
+When maintaining flows over longer periods of time it can save you time and
+interpretation errors if you add the comment node to your flows.
+
+There's 4 core benefits to adding the Comment node to your Node-RED flows:
+
+1. Improved readability: Comments can help to make flows easier to understand, especially when adding
+easy to miss context and explanation like implicit requirements of the flow. This can be especially
+helpful for complex flows with link nodes, or when you're editing in many tabs.
+1. Enhanced maintainability: Comments can help to make flows easier to maintain
+by providing a record of the flows purpose and functionality. This can be helpful
+when making changes to the flow or when troubleshooting problems, particularly
+if you collaborate with multiple team members.
+1. Improved debugging: Comments can help to make debugging easier by providing
+information about the expected behavior. This can be helpful when tracking down
+errors and identifying the source of problems, or reasoning errors when previously
+developing the flow.
+1. Increased documentation: Comments can be used to document the code, providing
+additional information about the code's purpose, functionality, and usage. Beyond
+what the flow or nodes do in the sequence, business logic and requirements documented
+next to the flow increased developer effeciency.
+
+The comment node can be added to any open space in the editor, and it's advised
+to add a comment to all [flow groups](/blog/2023/03/3-quick-node-red-tips-5/#3.-group-nodes-together-to-make-your-flows-easier-to-read).
+
+As the comment node will not take up more space if your write a larger comment,
+Node-RED allows you to be more explicit and elaborate further in your comments.
+ASCII diagrams or so do not distract from your flow, and as such you're encouraged
+to add these.
+
+### Examples
+
+![A comment node](./images/comment-node.png)
+
+When you double click the node, the full text is revealed:
+
+![Comment node editor](./images/comment-node-full-text.png)
+
+```json
+[{"id":"fc32e2266a87c07d","type":"comment","z":"1adbd550af387f9c","name":"Flow Explanation","info":"This comment can explain what nodes near to\nthis one are intended to do.\n\nComments are written in a WYSIWYG editor.","x":500,"y":320,"wires":[]}]
+```