add blurb in configuration doc page for overriding database locations

doc/configuration.rst

@@ -72,6 +72,24 @@ flag:
    ...
 
 
+When running an experiment via the ``beat`` application using the local
+executor (the default executor, also behind the ``--local`` flag), ``beat``
+will look into your configuration for any options set by the user that follow
+the format ``database/<db name>/<db version>``. ``beat`` expects that this
+option points to a string representing the path to the root folder of the
+actual database files for the given database.
+
+For example, the AT&T "Database of Faces" is available on the BEAT platform
+as the "atnt" database. The third version of the "atnt" database would be
+referenced as "atnt/3". The object "atnt/3" has a root folder defined on
+the BEAT platform already, and changing this locally would mean creating a
+new version of the database.
+Instead, you may override that path by setting the configuration option
+``database/atnt/3`` to your local path to the database files.
+Assuming your username is "user" and you extracted the database files to
+``~/Downloads/atnt_db``, you can set ``database/atnt/3`` to
+``/home/user/Downloads/atnt_db``, and ``beat`` will find the database files.
+
 You may explore different configuration options with the ``--help`` flag of
 ``beat config``: