And they don't need to look up frame- and focus-related documentation,
as long as they only need to select a window. What you needed is to
select another frame, select another window on that frame, and make
sure that frame is at the top of the z-order. That has very little to
do with selecting a window, and much more to do with frames and input
focus -- and those topics are described in the docs under their proper
subjects.
I could support some gentle notes in the ELisp manual's node
"Selecting Windows", where it already mentions frames. But doc
strings are inappropriate for references to almost unrelated subjects,
nor for general descriptions of complex topics.