Add back IndexReader.fields() -> Multi*, or discourage term vectors in some better way [LUCENE-4694]

Users can easily get term vectors from any indexreader, but not postings lists. this encourages them to do really slow things: like pulling term vectors for every single document.

this is really really so much worse than going through multifields or whatever.

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Migrated from [LUCENE-4694](https://issues.apache.org/jira/browse/LUCENE-4694) by Robert Muir (@rmuir), 1 vote, updated Mar 10 2013
Attachments: [LUCENE-4694.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-4694/LUCENE-4694.patch)

Start by reviewing the issue text and attached LUCENE-4694.patch, then locate IndexReader.fields() and the referenced Multi* APIs in the Java sources. The issue does not name files or tests, and work is not complete until the project has a decided API or alternative for avoiding inefficient term-vector access, with corresponding validation.

Written by the indexing model from the issue text.