Add a common pitfalls and mistakes to the reusable helpers recipe.

[igalklebanov]

Hey 👋

We only talk about the Do's, but not really emphasize the Dont's when it comes to reusable helper implementation.

The classics:

• string[] and string instead of literal/s.
• ('col1' | 'col2' | ...)[] instead of readonly ['col2'].
  etc. etc.

We have lots of data for this in Discord/GH issues, but none of that exists in our docs and in our llms.txt and llms-full.txt.

We obviously can't teach people TypeScript in a TypeScript library's docs, but we can link to concepts and/or give brief explanations.