Help: Document file(GENERATE) signature option order

In commit v3.1.0-rc1~484^2 (Help: Format and revise file() command
documentation, 2014-05-23) the signature of file(GENERATE) was
accidentally simplified too much and dropped specification of the
required argument ordering.  Restore the signature to make the order
clear.

Help/command/file.rst

@@ -247,7 +247,9 @@ the ``<format>`` and ``UTC`` options.
 
 ::
 
-  file(GENERATE <options>...)
+  file(GENERATE OUTPUT output-file
+       <INPUT input-file|CONTENT content>
+       [CONDITION expression])
 
 Generate an output file for each build configuration supported by the current
 :manual:`CMake Generator <cmake-generators(7)>`.  Evaluate