Some more explanation improvements

[slow-J]

Description

A recent Explanation change I done, (#16229) has led me to more explanation improvements.

I am thinking of looking at the following:

1. Testing gap in testExplainFailingOptionalClauses. It only asserts on the failingOptionals list, not the main subs list. Adding a subs.clear() into BooleanWeight doesn't fail any tests.
2. CheckHits.checkNoMatchExplanations is shallow. It only checks the root explanation's isMatch() == false and never looks at its sub-details. Nested non-match sub-explanations are never structurally verified.
3. Improve the KNN explain message. When testing BooleanWeight.explain prints failing optional clauses (when required) #16229, we saw some unclear output, see below.
   • PR merged: Add detail to KNN no-match explanations #16271

0.0 = no match on optional clause (track(knn=DocAndScoreQuery[1404635,...][0.7655482,...],0.7655482))
    0.0 = not in top 1 docs

4. TestComplexExplanations has a long-standing // :TODO: we really need more crazy complex cases.