If you have functions with documentation, you are already so close to a package that it seems to make sense to just add a bare bones DESCRIPTION and NAMESPACE file and you'll get everything you want from RStudio. Why are you against making it a lightweight package?
A package for personal use doesn't need to be anything more than an R script or two, some documentation (by roxygen or otherwise), and the two files mentioned above.
Help documentation is recognized automatically for any installed R package. I don't think you can get the benefits of help documentation and more descriptive autocompletion in RStudio without making it a simple package.