RE: doc of prompt change with minor mode for read-key-sequence?

[Drew Adams]

        It took me quite a while to discover where this second
        prompt was coming from. Because the translation happens
        at a low level, I couldn't figure out a way to debug it.
        By experimenting and searching source code, I found that
        "Foo: " comes from the name of the minor-mode keymap:
        (make-sparse-keymap "Foo"). (I don't know where the ": "
        is appended or in what code that takes place.)

    I am not quite sure why this is happening, but I can tell you
    something.  The "Foo" you specified in the keymap is its name as a
    menu.  It ought to be used if the map is used as a menu, either a
    mouse menu or a keyboard menu.

    I do not see why it would be used as a menu in this case,
    but apparently it is.

Right. I don't know why this happens either.

My concern is that this is incomprehensible to users. If there is a good
reason why this happens, then perhaps the prompt should make the meaning
more explicit somehow. And, it should be documented somewhere. That was my
point.

Does someone know what this is all about, what it is for? (And where in the
code it happens?)