minor #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 #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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