Add comments to docs that cover dev mode warnings

[arthurevans]

The shortlinks in #465 redirect to sections that explain specific issues that devmode warns about. There should be a comment in each of these locations so the hapless editor (ahem) knows to consider this factor rearranging docs.

We also probably need a policy about how to handle these over time. i.e., if Lit 3 eliminates a problem or adds a different solution for it, we still need to keep the Lit 2 shortlink (and associated doc) alive.

[arthurevans]

It may be overkill, but I want to add comments where we link to the main docs. Reopening to track those.