Some subclasses of Scorer do not honor the contract of DocsEnum.freq()

The behavior of Scorer.freq() is inconsistent across its subclasses:

• For TermScorer, the freq() method behaves just as DocsEnum.freq() specifies, i.e. the method returns the "term frequency in the current document".
• For BooleanScorer2, ConjunctionScorer, DisjunctionMaxScorer, DisjunctionSumScorer, and possibly other classes, the freq() method returns the number of clauses (in BooleanQuery or DisjunctionMaxQuery) that match the current document.

This difference makes the meaning of Scorer.freq()'s return value uncertain. To add to the uncertainty, given a Query, there seems to be no way of knowing which behavior takes effect (as that is not specified in the API reference) except by reading or running the code.

This issue might be related to LUCENE-4514.