Review/edit `dhcp4-srv.rst`, `dhcp6-srv.rst`, and `database-connectivity.rst` (MR !1430)

The following discussions from !1401 (merged) should be addressed:

• @wlodek started a discussion: (+3 comments)

  ```key-file`` looks wrong

• @wlodek started a discussion:

  that's no longer true, now we have just major version numbering and it's not possible to run kea with older version of schema (kea check at startup version and exit with an error when version is not up-to-date)

• @wlodek started a discussion:

  use `` instead of "

• @tomek started a discussion:

  A contribution of this size absolutely requires a Changelog.