automatically generate usage section for dataset documentation

[tobias-verbeke]

when documenting a dataset (@doctype data) and
if no @Usage tag is specified, the usage section in
the .Rd document should be populated using the
value of the @name tag, so if

@name someDataSet

is given in the roxygen comments, one would have

\usage{someDataSet}

in the resulting .Rd file.

Example from the datasets package
http://svn.r-project.org/R/trunk/src/library/datasets/man/ChickWeight.Rd

[hadley]

Fixed.