minor symfony#9141 Update doctrine.rst (tvlooy)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes symfony#9141).

Discussion
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Update doctrine.rst

my doctrine.yml had a ```charset: utf8mb4``` but my schema update generated tables with utf8. Seems like there is a separate configuration directive for the tables and this is not inherited from the database

Commits
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81f9051 Update doctrine.rst

Author: javiereguiluz

reference/configuration/doctrine.rst

@@ -31,7 +31,12 @@ Full Default Configuration
                         port:                 ~
                         user:                 root
                         password:             ~
+                        # charset of the database
                         charset:              ~
+                        # charset and collation of the tables. Not inherited from database
+                        default_table_options:
+                            charset:          ~
+                            collate:          ~
                         path:                 ~
                         memory:               ~