minor #8851 Add "url" key in dbal configuration (yordandv, weaverryan)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes #8851).

Discussion
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Add "url" key in dbal configuration

"url" key exist in default doctrine configuration created from flex (doctrine.yaml)

Commits
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577edd3 adding note to config
aa3f53d Add "url" key in dbal configuration

Author: weaverryan

reference/configuration/doctrine.rst

@@ -293,6 +293,8 @@ The following block shows all possible configuration keys:
                 user:                 user
                 password:             secret
                 driver:               pdo_mysql
+                # if the url option is specified, it will override the above config
+                url:                  mysql://db_user:db_password@127.0.0.1:3306/db_name 
                 # the DBAL driverClass option
                 driver_class:         MyNamespace\MyDriverImpl
                 # the DBAL driverOptions option