extend checkJavaDocs.py to methods,constants,fields [LUCENE-4324]

We have a large amount of classes in the source code, its nice that we have checkJavaDocs.py to ensure packages and classes have some human-level description.

But I think we need it for methods etc too. (it is also part of our contribution/style guidelines: http://wiki.apache.org/lucene-java/HowToContribute#Making_Changes)

The reason is that like classes and packages, once we can enforce this in the build, people will quickly add forgotten documentation soon after their commit when its fresh in their mind.

Otherwise, its likely to never happen.

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Migrated from [LUCENE-4324](https://issues.apache.org/jira/browse/LUCENE-4324) by Robert Muir (@rmuir), updated Aug 28 2012
Attachments: [LUCENE-4322.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-4324/LUCENE-4322.patch), [LUCENE-4324_crawl.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-4324/LUCENE-4324_crawl.patch)

Start with checkJavaDocs.py and review the linked LUCENE-4322.patch and LUCENE-4324_crawl.patch to understand the existing package and class checks. Extend the checker to cover methods, constants, and fields, then verify that the build enforces the documentation requirement described in the issue and contribution guidelines.

Written by the indexing model from the issue text.