[numeric.limits] use "primary template" and "value-initialized"

Improve the language used to describe the primary template and the
values of its members.

Author: jwakely

source/support.tex

@@ -885,11 +885,14 @@
 constant expressions.
 
 \pnum
-The default
-\tcode{numeric_limits<T>}
-template shall have all members, but with 0 or
-\tcode{false}
-values.
+For the
+\tcode{numeric_limits}
+primary template, all data members are value-initialized and all
+member functions return a value-initialized object.
+\begin{note}
+This means all members have zero or \tcode{false} values
+unless \tcode{numeric_limits} is specialized for a type.
+\end{note}
 
 \pnum
 Specializations shall be provided for each