Rollup merge of #121732 - Voultapher:improve-assert_matches-documentation, r=cuviper

Improve assert_matches! documentation

This new documentation tries to limit the impact of the conceptual pitfall, that the if guard relaxes the constraint, when really it tightens it. This is achieved by changing the text and examples. The previous documentation also chose a rather weird and non-representative example for the if guard, that made it needlessly complicated to understand.

Author: matthiaskrgr