Creating placeholder constraint rst docs for deprecated usage

Author: Tim Stamp

reference/constraints/False.rst

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+False
+=====
+
+.. caution::
+
+    The ``False`` constraint is deprecated since Symfony 2.7
+    and will be removed in Symfony 3.0. Use the ``IsFalse`` constraint instead.
+
+.. include:: /reference/constraints/IsFalse.rst

reference/constraints/Null.rst

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+Null
+====
+
+.. caution::
+
+    The ``Null`` constraint is deprecated since Symfony 2.7
+    and will be removed in Symfony 3.0. Use the ``IsNull`` constraint instead.
+
+.. include:: /reference/constraints/IsNull.rst

reference/constraints/True.rst

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+True
+====
+
+.. caution::
+
+    The ``True`` constraint is deprecated since Symfony 2.7
+    and will be removed in Symfony 3.0. Use the ``IsTrue`` constraint instead.
+
+.. include:: /reference/constraints/IsTrue.rst