Complex queries in term frequency visualisation

Various types of complex queries are not handled well in the frequency visualisation. This has been the case since the feature was built, mostly because it was not a priority at the time. This should be addressed because results can be misleading.

I'm already working on this as part of #1718, this issue is just for documentation. These issues are only about counting matches within documents, not about the selection of documents.

Known issues:
- Negated terms, fuzzy search, and proximity search are not supported and can give unexpected results
- Segmentation ignores parentheses, which can be a problem in combination with phrase search
- Escaping (quotation) characters is not supported

Proximity search will not be fixed at present, which is acceptable, but there should be a warning if the user tries to use the visualisation with a proximity query.

Review the frequency visualisation work in #1718 first, since this issue says implementation is already underway there. Use the listed cases to verify the scope: unsupported negated, fuzzy, proximity, parenthesised phrase, and escaped-quote queries, with a warning required for proximity queries.

Written by the indexing model from the issue text.