Improve macOS-specific documentation

[hturner]

Another suggestion from @s-u:

What I would welcome is to identify and improve specifically macOS-related documentation: coming from the user's point of view -- what is not covered or confusing for Mac users in the current documentation (taking into account all three: CRAN page, mac.R-project.org and R-admin manual). This is more crowd-sourcing of issues than real work, but it seems that it would be a good exercise while everyone is around and I bet lots of people have Macs.