Auto merge of #114901 - compiler-errors:style-guide-wc, r=calebcartwright

Amend style guide section for formatting where clauses in type aliases

This PR has two parts:
1. Amend wording about breaking before or after the `=`, which is a style guide bugfix to align it with current rustfmt behavior.
2. Explain how to format trailing (#89122) where clauses, which are preferred in both GATs (#90076) and type aliases (#114662).

r? `@joshtriplett`

Author: bors