In check's call to document: don't use clean=TRUE

When running `check()`, it calls `document(clean=TRUE)`, which deletes existing Rd files. This can be a problem for packages that have some or all documentation that's NOT generated by roxygen, especially if the developer doesn't have those files checked into version control.

This patch removes the `clean=TRUE`. 

Another possibility is to make this an option to `check()`, like `cleandoc = FALSE` or something like that. If that's done, then `release()` should also have that option.