RE: emacs-25 f708cb2: Clarify doc string of 'transpose-sexps'

[Drew Adams <drew.adams@oracle.com>]

> > It is not unreasonable to be clear and brief.  I repeat:
> > if the doc means "the innermost sexp containing point" then
> > it should just say that.  That's not being pedantic; it's
> > being clear.
> 
> So is the following (combining Eli's changes with my correction and
> your clarification) then clear enough?
> 
...
> If the innermost sexp containing point is a list or string, you
> cannot transpose that sexp with another sexp.
...

LGTM.

But as one reader I would still want to know what "cannot"
means here - what happens if you try?  Eli has replied in the
bug thread that there are several possible behaviors (effects),
so I guess we can't usefully say what it means.  Dommage.

Here's a thought: Are any of the behaviors resulting from
trying that useful?  If so, then they should perhaps be
documented.  And if not, why don't we check for that
situation and raise an error if you try?

It is really not great to tell someone they "cannot" or
"must not" etc. do something without raising an error that
really enforces "cannot" etc.

Sure, if you try, the result won't transpose the sexps, so
technically you "cannot" transpose sexps in this case.  But
I'd sooner see a clear error than just some unpredictable
behavior.  (Unpredictable from reading the doc.  There are
multiple possibilities, and we won't be detailing them in
the doc.)