minor symfony#12419 Highlighting possibilty to use FQCN as type value. (ktherage)

This PR was submitted for the 4.3 branch but it was squashed and merged into the 3.4 branch instead (closes symfony#12419).

Discussion
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Highlighting possibilty to use FQCN as type value.

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Commits
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24ad431 Highlighting possibilty to use FQCN as type value.

Author: javiereguiluz

reference/constraints/Type.rst

@@ -106,6 +106,10 @@ This will check if ``firstName`` is of type ``string`` and that ``age`` is an
             }
         }
 
+.. tip::
+
+    Type value can be class's or interface's fully qualified class name
+
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