Change naming of variables/apis from docid to docnum [LUCENE-7453]

In [SOLR-9528](https://issues.apache.org/jira/browse/SOLR-9528) a suggestion was made to change `docid` to `docnum`. The reasoning for this is most notably that `docid` has a connotation about a persistent unique identifier (eg like `_id` in elasticsearch or `id` in solr), while `docid` in lucene is currently some local to a segment, and not comparable directly across segments.

When I first started working on Lucene, I had this same confusion. `docnum` is a much better name for this transient, segment local identifier for a doc. Regardless of what solr wants to do in their api (eg keeping _docid_), I think we should switch the lucene apis and variable names to use docnum.

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Migrated from [LUCENE-7453](https://issues.apache.org/jira/browse/LUCENE-7453) by Ryan Ernst (@rjernst), 1 vote, updated Oct 04 2016

Start by searching the Lucene APIs and variables for the `docid` name and read the surrounding definitions and callers. Determine the complete scope of the terminology change, then verify that the affected APIs, callers, and existing checks consistently use `docnum` without changing its segment-local meaning.

Written by the indexing model from the issue text.