queryparsersyntax.html escaping section needs beefed up [LUCENE-1000]

the query syntax documentation is currently lacking several key pieces of info:
1) that unicode style escapes are valid
2) that any character can be escaped with a backslash, not just special chars.

..we should probably beef up the "Escaping Special Characters" section

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Migrated from [LUCENE-1000](https://issues.apache.org/jira/browse/LUCENE-1000) by Chris M. Hostetter (@hossman), 1 vote, updated May 09 2016
Linked issues:
- #2199

Start with queryparsersyntax.html and review the "Escaping Special Characters" section. Document that Unicode-style escapes are valid and that a backslash can escape any character, then confirm the section clearly covers both points.

Written by the indexing model from the issue text.