Links are very tricky, so I'll put in some links here:
Link to a function: roxygenize()
.
Link to an object: roxygenize (we just treat it like an object here.
Link to another package, function: devtools::document()
.
Link to another package, non-function: devtools::document.
Link with link text: this great function or that great function.
In another package: and this one.