Clarify realm and dbmodules configuration docs

In kdc_conf.rst, add examples showing how to configure a realm
parameter and a database parameter.  Document that the default DB
configuration section is the realm name, and use that in the example.
Move the db_module_dir description to the end of the [dbmodules]
documentation since it is rarely used and could confuse a reader about
the usual structure of the section.

ticket: 7759 (new)
target_version: 1.12
tags: pullup