Document module configuration directory

Added documentation for dbfwfilter and mentioned the new directory in the
release notes. The configuration guide also gives an example of how the
path parameters are resolved.

Documentation/Filters/Database-Firewall-Filter.md

@@ -28,7 +28,10 @@ The database firewall filter has one mandatory parameter, `rules`.
 
 #### `rules`
 
-Absolute path to a file with the rule definitions in it. The file should be readable by the user MariaDB MaxScale is run with.
+A path to a file with the rule definitions in it. The file should be readable by
+the user MariaDB MaxScale is run with. If a relative path is given, the path is
+interpreted relative to the module configuration directory. The default module
+configuration directory is _/etc/maxscale.modules.d_.
 
 #### `action`
 

Documentation/Getting-Started/Configuration-Guide.md

@@ -394,6 +394,14 @@ Configure the directory where module configurations are stored. Path arguments
 are resolved relative to this directory. This directory should be used to store
 module specific configurations e.g. dbfwfilter rule files.
 
+Any configuration parameter that is not an absolute path will be interpreted as
+a relative path. The relative paths use the module configuration directory as
+the working directory.
+
+For example, the configuration parameter `file=my_file.txt` would be interpreted
+as `/etc/maxscale.cnf.d/my_file.txt` whereas `file=/home/user/my_file.txt` would
+be interpreted as `/home/user/my_file.txt`.
+
 ```
 module_configdir=/var/lib/maxscale/
 ```

Documentation/Release-Notes/MaxScale-2.1.0-Release-Notes.md

@@ -22,6 +22,23 @@ Please see the
 [Configuration Guide](../Getting-Started/Configuration-Guide.md#configuration)
 for details.
 
+### Module configurations
+
+MaxScale 2.1 introduces a new directory for module configurations. This new
+directory can be used to store module specific configuration files.
+
+Any configuration parameter that accepts a path will also support relative
+paths. If a relative path is given, the path is interpreted relative to
+the module configuration directory. The default value is
+_/etc/maxscale.modules.d_.
+
+For example, the `dbfwfilter` rule files could be stored in
+_/etc/maxscale.modules.d/my_rules.txt_ and referred to with
+`rules=my_rules.txt`.
+
+For more details, refer to the documentation of _module_configdir_ in the
+[Configuration Guide](../Getting-Started/Configuration-Guide.md)
+
 ### Logging
 
 Before version 2.1.0, MaxScale created in the log directory a log file