Prominently document that overlong bitshifts cause undefined behavior

Per #10183.  This is a soundness hole and it seems unlikely to be fixed before 1.0.   It needs to be clearly documented in the reference and elsewhere so people know that they have to write their code carefully to avoid it.  See http://www.postgresql.org/docs/9.1/static/indexes-types.html caveats on hash indexes for an example of how to document major bugs in this way.
