Clearly document the limit for maximum number of documents in a single index [LUCENE-4104]

Although the "int" in a number of APIs strongly suggests the approximate limit to the number of documents than can exist in a single Lucene index, it would be useful to have the specific number more clearly documented.

My reading suggests that the limit is 2^31-2 so that the count of documents, 0 to 2^31-2, will fit in an int as Integer.MAX_VALUE or 2^31-1 or 2,147,483,647.

Symbolic definitions of the maximum document number and maximum number of documents, as well as the first document number should also be provided.

A subsequent issue will be to detect and throw an exception when that limit is exceeded.

---
Migrated from [LUCENE-4104](https://issues.apache.org/jira/browse/LUCENE-4104) by Jack Krupansky

Start by locating the Lucene APIs that expose document numbers and counts, then verify the proposed 2^31-2 limit and the requested symbolic definitions. Done means the maximum document number, maximum document count, and first document number are clearly documented; the issue does not name specific files or tests.

Written by the indexing model from the issue text.