Call me dense, but guessing the semantics of the return value
of make-glyph by following a link to a different function and
then seeing how this function may deal with certain
conditions possibly arising from completely different
functions, making it somewhat probably that make-glyph might
be passing them on in some manner, or not detect them, or
whatever, is not _exactly_ what I'd call totally obvious.
Well, if you actually read the docs you see that in the second paragraph of
`make-glyph' it tells you that if you want to understand the format of the
SPEC parameter to `make-glyph', go see `make-image-specifier'. Since your
initial question was about the behavior of different kinds of SPEC
parameters, it doesn't take an einstein to figure out that the place that
describes spec parameters in detail might say something about their
behavior.
Ben