Doc: in back branches, don't call it a row constructor if it isn't really.

Doc: in back branches, don't call it a row constructor if it isn't really.

Before commit 906bfcad7, we were not actually processing the righthand
side of a multiple-column assignment in UPDATE as a row constructor:
it was just a parenthesized list of expressions.  Call it that rather
than risking confusion by people who would expect the documented behaviors
of row constructors to apply.

Back-patch to 9.5; before that, the text correctly described the construct
as a "list of independent expressions".

Discussion: <16288.1479610770@sss.pgh.pa.us>