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.

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I could support some gentle notes in the ELisp manual's node

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"Selecting Windows", where it already mentions frames.  But doc

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strings are inappropriate for references to almost unrelated subjects,

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nor for general descriptions of complex topics.