minor #12325 Reworded the docs about storing sessions in database (javiereguiluz)

This PR was squashed before being merged into the 4.4 branch.

Discussion
----------

Reworded the docs about storing sessions in database

This is part of our ongoing effort to reword all Doctrine related docs.

I'll summon the power of the Symfony community to review this:

* @dkarlovi could you please review that the section about RedisSessionHandler is correct? Thanks!
* @stof could you please review the section about relational databases? Related to #12324. Thanks!
* @alcaeus could you please review the section about MongoDB? I have some problems about the config needed to pass the MongoDB connection to MongoDBSessionHandler. Thanks!

The file to review is the new file `session/database.rst`. Here's how it looks rendered: https://github.com/javiereguiluz/symfony-docs/blob/b2ef983b79110cff8f85abeeffe78aa18f6bf31a/session/database.rst

Thank you all!

Commits
-------

500d961 Reworded the docs about storing sessions in database

Author: javiereguiluz

_build/redirection_map

@@ -479,3 +479,5 @@
 /components/translation/custom_formats https://github.com/symfony/translation
 /components/translation/custom_message_formatter https://github.com/symfony/translation
 /components/routing https://github.com/symfony/routing
+/doctrine/pdo_session_storage /session/database
+/doctrine/mongodb_session_storage /session/database

doctrine.rst

@@ -853,10 +853,9 @@ Learn more
     doctrine/custom_dql_functions
     doctrine/dbal
     doctrine/multiple_entity_managers
-    doctrine/pdo_session_storage
-    doctrine/mongodb_session_storage
     doctrine/resolve_target_entity
     doctrine/reverse_engineering
+    session/database
     testing/database
 
 .. _`Doctrine`: https://www.doctrine-project.org/