feature #4150 Added the schema_filter option to the reference (peterrehm)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4150).

Discussion
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Added the schema_filter option to the reference

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | ?
| Fixed tickets | -

Shall the optional setting be added to XML as well? How?
I am unsure if it is actually to which version this relates to the DoctrineBundle.

Relates to doctrine/migrations#101

Commits
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20cba4e Added the schema_filter option to the reference

Author: weaverryan

reference/configuration/doctrine.rst

@@ -21,6 +21,9 @@ Full default configuration
                     some_custom_type:
                         class:                Acme\HelloBundle\MyCustomType
                         commented:            true
+                # If enabled all tables not prefixed with sf2_ will be ignored by the schema
+                # tool. This is for custom tables which should not be altered automatically.
+                #schema_filter:        ^sf2_ 
 
                 connections:
                     default: