[DOCS] Clarifies the effect of per-field boosting (#63733) (#63744)

The original description of per-field boosting is incorrect. Boosting a
field does not imply that it is more important relative to other fields.
It simply means that the score is multiplied by the supplied boost
value. Due to the differences in each field's term and document
statistics, it's not possible to imply relative importance of fields
based on the per-field boost value alone.

Co-authored-by: Josh Devins <josh.devins@elastic.co>

docs/reference/query-dsl/multi-match-query.asciidoc

@@ -59,7 +59,8 @@ GET /_search
 }
 --------------------------------------------------
 
-<1> The `subject` field is three times as important as the `message` field.
+<1> The query multiplies the `subject` field's score by three but leaves the
+`message` field's score unchanged.
 
 If no `fields` are provided, the `multi_match` query defaults to the `index.query.default_field`
 index settings, which in turn defaults to `*`. `*` extracts all fields in the mapping that