[basic.types] Change redundant normative wording into note

Author: sdkrystian

source/basic.tex

@@ -4740,8 +4740,10 @@
 object type is unknown.}
 Incompletely-defined object types and \cv{}~\tcode{void} are
 \defnx{incomplete types}{type!incomplete}\iref{basic.fundamental}.
-Objects shall not be defined to have an
-incomplete type.
+\begin{note}
+Objects cannot be defined to have an
+incomplete type\iref{basic.def}.
+\end{note}
 
 \pnum
 A class type (such as ``\tcode{class X}'') might be incomplete at one