Fix doc for full text search distance operator.
Commit
028350f619 changed its behavior from "at most" to "exactly", but
forgot to update the documentation. Backpatch to 9.6.
Patch by Justin Pryzby, per Yaroslav Schekin's report.
Discussion: https://www.postgresql.org/message-id/
20201005191922.GE17626%40telsasoft.com