Update Doctrine MongoDB docs with new mapping configuration details.

Author: beberlei

guides/doctrine/mongodb-odm/configuration.rst

@@ -33,6 +33,59 @@ If you wish to use memcache to cache your metadata, you need to configure the
             port: 11211
             instance_class: Memcache
 
+Mapping Configuration
+~~~~~~~~~~~~~~~~~~~~~
+
+Explicit definition of all the mapped documents is the only necessary configuration for the ODM and there
+are several configuration options that you can control. The following configuration options exist
+for a mapping:
+
+- ``type`` One of "annotations", "xml", "yml", "php" or "static-php". This specifies which type
+  of metadata type your mapping uses.
+- ``dir`` Path to the mapping or entity files (depending on the driver). If this path is relative
+  it is assumed to be relative to the bundle root. This only works if the name of your mapping
+  is a bundle name. If you want to use this option to specifiy absolute paths you should prefix
+  the path with the kernel parameters that exist in the DIC (for example %kernel.dir%).
+- ``prefix`` A common namespace prefix that all documents of this mapping share. This prefix
+  should never conflict with prefixes of other defined mappings otherwise some of your documents cannot be found
+  by Doctrine. This option defaults to the bundle namespace + `Documents`, for example for an
+  application bundle called "Hello" prefix would be "Application\Hello\Documents".
+- ``alias`` Doctrine offers a way to alias document namespaces to simpler, shorter names to be used
+  inqueries or for Repository access.
+- ``is_bundle`` This option is a derived value from ``dir`` and by default is set to true if dir is relative
+  proved by a ``file_exists()`` check that returns false. It is false if the existance check returns true.
+  In this case an absolute path was specified and the metadata files are most likely in a directory outside of
+  a bundle.
+
+To avoid having to configure lots of information for your mappings you should follow these conventions:
+
+1. Put all your entities in a directory Documents/ inside your bundle. For example "Application/Hello/Documents/".
+2. If you are using xml, yml or php mapping put all your configuration files into the
+   "Resources/config/doctrine/metadata/doctrine/mongodb/" directory sufficed with dcm.xml, dcm.yml or dcm.php
+   respectively.
+3. Annotations is assumed if an "Documents/" but no "Resources/config/doctrine/metadata/doctrine/mongodb/"
+   directory is found.
+
+The following configuration shows a bunch of mapping examples:
+
+.. code-block:: yaml
+
+    doctrine.mongodb:
+        mappings:
+            MyBundle1: ~
+            MyBundle2: yml
+            MyBundle3: { type: annotation, dir: Documents/ }
+            MyBundle4: { type: xml, dir: Resources/config/doctrine/mapping }
+            MyBundle5:
+                type: yml
+                dir: my-bundle-mappings-dir
+                alias: BundleAlias
+            doctrine_extensions:
+                type: xml
+                dir: %kernel.dir%/../src/vendor/DoctrineExtensions/lib/DoctrineExtensions/Documents
+                prefix: DoctrineExtensions\Documents\
+                alias: DExt
+
 Multiple Connections
 ~~~~~~~~~~~~~~~~~~~~