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

Before commit 906bfca, 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: <[email protected]>

Author: tglsfdc

doc/src/sgml/ref/update.sgml

@@ -422,9 +422,9 @@ UPDATE films SET kind = 'Dramatic' WHERE CURRENT OF c_films;
    According to the standard, the source value for a parenthesized sub-list of
    column names can be any row-valued expression yielding the correct number
    of columns.  <productname>PostgreSQL</productname> only allows the source
-   value to be a parenthesized list of expressions (a row constructor) or a
+   value to be a parenthesized list of expressions or a
    sub-<literal>SELECT</>.  An individual column's updated value can be
-   specified as <literal>DEFAULT</> in the row-constructor case, but not
+   specified as <literal>DEFAULT</> in the list-of-expressions case, but not
    inside a sub-<literal>SELECT</>.
   </para>
  </refsect1>