Clarify R terms to avoid common mistake

This merge request makes a clearer distinction between the terms "package" and "library" in R to avoid a common mistake of thinking that they mean the same in R. As this document is targeted at new R users, it is easier to teach correctly in the first place than to reteach it afterward.

Link to official R documentation: https://cran.r-project.org/doc/manuals/R-exts.html#Creating-R-packages (Especially part "A package is not a library[...]").