[A Github issue](https://github.com/angular/angular/issues/43821) about an arcane-sounding Forms error is one of the repo's top-ten most visited pages. This converts the error to `RuntimeErrorCode` and adds a dedicated guide to explain how to solve it.
PR Close#47969
Replace `new Error()` in a forms Validators function with `RuntimeError`, for better tree-shakability. Also, improve the error messages, and add documentation.
PR Close#46537
Runtime error codes in the Core, Common and Forms packages were not included into the `public-api` group reviews. This commit creates the necessary golden files to keep track of further changes in the runtime codes.
This is a followup from https://github.com/angular/angular/pull/44398#issuecomment-1006910976.
PR Close#44677
