Miscellaneous documentation cleanups

This MR is a forward-port of the documentation cleanups which were prepared in the course of working on BIND 9.18.0 documentation. Wherever that MR inserted the string "BIND 9.18" into the documentation, this one inserts "BIND 9.19" instead as it is targeted at main. Release note changes present in the original MR were intentionally excluded; release notes for BIND 9.19.0 will be dealt with in due course.