[Content]: Eager Loading should mention what field types and properties benefit/support it

Product

Other/Multiple/Not Sure

Description

We aren't currently explicitly listing the fields and/or properties of elements that can be eager-loaded.

This may have to be "generic," but should still be front-and-center, i.e. "Only properties and custom fields that reference other elements can be eager-loaded."

Additionally, not all fields and/or related elements need to be eager-loaded. We can cover situations where you are apt to see performance improvements vs situations where you won't see any benefit.

This may include a mention of the eager-loading-in-place service method:

{% do craft.app.elements.eagerLoadElements(
  className(entry),
  [entry],
  [
     ...
  ]
) %}

Start from the existing eager-loading documentation; the issue does not name a file or page. Update the relevant content to explain which field types and properties support eager loading, when it improves performance, and whether to include the shown eagerLoadElements() example. Done means these limitations and use cases are clearly documented.