Bug 18773 - grid::dataViewport() has minor bugs in error messages and help

[hturner]

This is a good first issue making small changes to the code and documentation of grid::dataViewport(). The changes are already proposed in Bug 18773.

Todo:

• Follow the R Dev Container documentation to start the R Dev Container in GitHub Codespaces or on Gitpod.
• Follow the instructions to build R.
• Follow the contribution workflow to make the suggested changes to the R and Rd files: (src/library/grid/R/viewport.R and src/library/grid/man/dataViewport.Rd, respectively), rebuild R and check the changes.
• Follow the guide to creating a patch file to create a patch file. If you have an account on R's Bugzilla, attach the patch with a comment, otherwise share it here to be added later.