minor symfony#7092 Added note on ODM id notation being different (toby-griffiths, javiereguiluz)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes symfony#7092).

Discussion
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Added note on ODM id notation being different

As the ODM YML notation for the `id` field is different, I've added a note, as I spent ages wondering why it wasn't working for me.

Commits
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0c07271 Removed the proposed note and updated the title
21733af Added note on ODM id notation being different

Author: xabbuh

doctrine.rst

@@ -1,8 +1,8 @@
 .. index::
    single: Doctrine
 
-Databases and Doctrine
-======================
+Databases and the Doctrine ORM
+==============================
 
 One of the most common and challenging tasks for any application
 involves persisting and reading information to and from a database. Although