Merge pull request #5438 from ethereum/docs-faq-types-undefined

DOCS: Move undefined FAQ item

docs/types.rst

@@ -13,6 +13,11 @@ Solidity provides several elementary types which can be combined to form complex
 In addition, types can interact with each other in expressions containing
 operators. For a quick reference of the various operators, see :ref:`order`.
 
+The concept of "undefined" or "null" values does not exist in Solidity, but newly
+declared variables always have a :ref:`default value<default-value>` dependent
+on its type. To handle any unexpected values, you should use the :ref:`revert function<assert-and-require>` to revert the whole transaction, or return a
+tuple with a second `bool` value denoting success.
+
 .. index:: ! value type, ! type;value
 
 Value Types