Boost using caret sign

I read on this website that I can use the caret sign ("^") to boost a field in a multi_match query, like so:

GET /bookdb_index/_search
  "query": {
    "multi_match": {
      "query": "guide",
      "fields": ["summary", "authors", "publisher", "title^20"]

But this is described nowhere in the Elastic documentation. Does anybody know something about it?

In addition, I noticed that the page on Boosting in the ES guide is very faulty:

It looks like it's a mix of index and query boosting.
If I find the time, I will correct it and create a PR.

Indeed the boosting is not really explaining in the match or multi match query from a quick peek. There is a small explanation however at that also applies to match/multi-match.

Documentation PRs are always appreciated!