Merge pull request #12791 from ethereum/usingForDocAtFileLevel

[DOC] Using for at file level.

Author: chriseth

docs/layout-of-source-files.rst

@@ -3,8 +3,8 @@ Layout of a Solidity Source File
 ********************************
 
 Source files can contain an arbitrary number of
-:ref:`contract definitions<contract_structure>`, import_ directives,
-:ref:`pragma directives<pragma>` and
+:ref:`contract definitions<contract_structure>`, import_ ,
+:ref:`pragma<pragma>` and :ref:`using for<using-for>` directives and
 :ref:`struct<structs>`, :ref:`enum<enums>`, :ref:`function<functions>`, :ref:`error<errors>`
 and :ref:`constant variable<constants>` definitions.