[class.pre] [class.union] Clean up definition of 'union'.

Move primary definition from [class.pre] to [class.union]. Add note to
[class.pre] specifying where to look for the meaning of the class-key.
Move note on aggregate classes from [class.pre] to [class.prop].

Author: zygoloid

source/classes.tex

@@ -99,6 +99,14 @@
 \grammarterm{class-head-name} of the
 definition shall not begin with a \grammarterm{decltype-specifier}.
 
+\pnum
+\begin{note}
+The \grammarterm{class-key} determines
+whether the class is a union\iref{class.union} and
+whether access is public or private by default\iref{class.access}.
+A union holds the value of at most one data member at a time.
+\end{note}
+
 \pnum
 If a class is marked with the \grammarterm{class-virt-specifier} \tcode{final} and it appears
 as a \grammarterm{class-or-decltype} in a \grammarterm{base-clause}\iref{class.derived},
@@ -138,14 +146,6 @@
 can be defined by the user; see~\ref{over.oper}.
 \end{note}
 
-\pnum
-A \defn{union} is a class defined with the \grammarterm{class-key}
-\tcode{union};
-it holds at most one data member at a time\iref{class.union}.
-\begin{note}
-Aggregates of class type are described in~\ref{dcl.init.aggr}.
-\end{note}
-
 \rSec1[class.prop]{Properties of classes}
 
 \pnum
@@ -283,6 +283,11 @@
 \end{codeblock}
 \end{example}
 
+\pnum
+\begin{note}
+Aggregates of class type are described in~\ref{dcl.init.aggr}.
+\end{note}
+
 \rSec1[class.name]{Class names}
 \indextext{definition!class name as type}%
 \indextext{structure tag|see{class name}}%
@@ -3080,6 +3085,10 @@
 \rSec1[class.union]{Unions}%
 \indextext{\idxcode{union}}
 
+\pnum
+A \defn{union} is a class defined with the \grammarterm{class-key}
+\tcode{union}.
+
 \pnum
 In a union,
 a non-static data member is \defnx{active}{active!union member}