The code completion in RStudio is great, and I really like how a popover appears to describe the arguments for the function inputs. For example, if one types
To add to Hadley's answer in the comments, Rstudio is mining specific portions of the help files to generate the helper window. Specifically, tabbing before the parentheses brings up the "Usage" and "Description" sections and tabbing inside the parentheses or after a comma brings up the "Arguments" section. Therefore, not only does a package need to be made, but the help files must be generated to take advantage of this feature.