Markdown notation can be used to create links to other manual pages. There are six kinds of links:
[...]
3. Link to a function from another package: [pkg::func()].
How would I do this with "special" functions like magrittr::%>% or base::%in%?
I have tried [base::%in%] and [base::`%in%`] among others, but to no avail.
At the bottom of the documentation page you link to it says,
Links to operators
Links to operators or objects that contain special characters, do not work currently. E.g. to link to the %>% operator in the magrittr package, instead of [magrittr::%>%] , you will need to use the Rd notation: \code{\link[magrittr]{\%>\%}} .