doc: revise intro sentence for assert

Simplify language, remove redundancy ("tests that can be used to
test") and remove wordy passive voice ("that can be used to" replaced
with "for").

PR-URL: #28226
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: Trott

doc/api/assert.md

@@ -4,8 +4,8 @@
 
 > Stability: 2 - Stable
 
-The `assert` module provides a simple set of assertion tests that can be used to
-test invariants. The module provides a recommended [`strict` mode][] and a more
+The `assert` module provides a set of assertion functions for verifying
+invariants. The module provides a recommended [`strict` mode][] and a more
 lenient `legacy` mode.
 
 For more information about the used equality comparisons see