Clarify point in some jsonfuncs.c operator functions

The danger of this error message causing confusion is quite acute if we
assume that expressional indexing will be used extensively, because the
restriction on, say, calling the "jsonb -> text" operator only on what
jsonb internally knows to be an object effectively makes every such
expressional index a constraint on same (at least without some further
care).

Note this general risk in the documentation, too.