[TASK] Comments: document that todo notes are dropped on render

[CybotTM]

Adds a short section that todo notes must not remain in published documentation — resolve them before merging or demote them to author-only comments.

[linawolf]

I'd drop the "or write them as plain comments" alternative. I implemented
.. todo:: myself — not rendering it is by design, not a quirk. It's meant
to be an internal todo comment, and a better one than a bare .. comment,
since todo:: is a distinct, greppable string (.. alone also starts every
directive and anchor).

I'd flip the guidance: keep using .. todo:: for internal notes, just warn
that it gives no visual reminder in the rendered output — grep for todo::
before merging rather than relying on noticing it in a preview.

[CybotTM]

Flipped in baf5a8d, thank you — you are right that the plain-comment alternative was the wrong advice. The section now says to keep using .. todo:: for internal notes, gives your reason for preferring it over a bare .. comment (a distinct, greppable string, where .. also starts every directive and anchor), and warns only about the consequence: no visual reminder in the rendered page, so search for todo:: before merging. make test-docs renders clean.