Link to or document decorator equivalents

Nobody has claimed this yet.

Non-decorator users often wonder how to implement the functionality of our decorators in plain JS. We have this information for many decorators - `@property` equivalent is talked about in *Reactive properties*, `@query` equivalent is talked about in *Shadow DOM* - but we might not have it for all decorators and the information is distributed in the topical docs.

It'd probably be useful if we repeated the equivalents or linked to the topics from the decorator docs.

This brings up an issue that we don't have guides on the decorators, we only have them mentioned in particular topics or have API docs. We possibly just want links from API docs then.

Start by reviewing the existing decorator API docs and the topical docs for Reactive properties and Shadow DOM. Identify which decorator equivalents are already documented, then link or repeat that information consistently; done means non-decorator users can find the plain-JavaScript equivalent for each supported decorator.

Written by the indexing model from the issue text.