doc/user: improve builtin cluster descriptions

[benesch]

This brings the descriptions of the builtin clusters in line with what we're putting in the console (MaterializeInc/console#2587).

Motivation

• This PR refactors existing documentation.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:
  • n/a

[morsapaes]

Suggested change

	* You can run `SELECT` or `SUBSCRIBE` queries in this cluster as long
	as you only reference objects in the [system catalog
	objects](https://materialize.com/docs/sql/system-catalog/).
	* You can run `SELECT` or `SUBSCRIBE` queries in this cluster as long
	as you only reference [system catalog](https://materialize.com/docs/sql/system-catalog/) objects.

[benesch]

Ugh, thanks!