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.
Merge topic 'doc-file-GENERATE-order' into next

fe21580b Help: Document file(GENERATE) signature option order