Since OS X 10.12 it has been named macOS so it makes sense
to reference it in documentation as such, even when it
sometimes refers to older versions which were called (Mac) OS X.
This is not relevant any longer, all supported macOS versions support
x86-64 binaries and the latest ones don't support anything else, so
there is no need to create i386 binaries any more (and PPC ones don't
work since ages).
Don't explain how to install the library first only to say that it
shouldn't be done afterwards.
Also separate the discussion of installing and distributing the
libraries.
Closes#16316.
