Add a very specific hint for the case that we're unable to locate a function

Add a very specific hint for the case that we're unable to locate a function
matching a call like f(x, ORDER BY y,z).  It could be that what the user
really wants is f(x,z ORDER BY y).  We now have pretty conclusive evidence
that many people won't understand this problem without concrete guidance,
so give it to them.  Per further discussion of the string_agg() problem.