minor #7904 Fix incoherent utf8mb4 collation in Doctrine setup (marcverney)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7904).

Discussion
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Fix incoherent utf8mb4 collation in Doctrine setup

The "Setting up the Database to be UTF8" section of the "Databases and the Doctrine ORM" page recommends the following default collation setup:

- in MySQL's my.cnf: `collation-server = utf8mb4_general_ci`
- and/or in Symfony's config.yml: `collate: utf8mb4_unicode_ci`

As a reader I am confused by this difference. If there's a reason for it, I think it should be explained in the text. On the other hand, if there is none, both lines should probably reference `utf8mb4_unicode_ci` (reason: http://stackoverflow.com/a/766996/2516943). This PR intends to fix the latter case.

Commits
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c27f16a Fix incoherent ut8mb4 collation in Doctrine setup

Author: xabbuh

doctrine.rst

@@ -142,7 +142,7 @@ can automatically generate an empty ``test_project`` database for you:
 
         [mysqld]
         # Version 5.5.3 introduced "utf8mb4", which is recommended
-        collation-server     = utf8mb4_general_ci # Replaces utf8_general_ci
+        collation-server     = utf8mb4_unicode_ci # Replaces utf8_unicode_ci
         character-set-server = utf8mb4            # Replaces utf8
 
     We recommend against MySQL's ``utf8`` character set, since it does not