Improve XML Query Parser documentation

The xmlparser (XML Query Parser) documentation at https://lucene.apache.org/solr/guide/7_4/other-parsers.html#xml-query-parser is very shallow. This complex query parser needs in-depth documentation of all available query types, what elements are allowed inside each other, and a detailed tag-element reference for what options are allowed. There should also be a handful examples to cover typical use cases, and an explanation on implications, such as getting highlighting to work etc.