[expr.prim.lambda] Move syntactic restriction on decl-specifier-seq to the front.

Per CWG guidance.

Author: jensmaurer

source/expressions.tex

@@ -634,6 +634,11 @@
 A closure object behaves like a function
 object~(\ref{function.objects}).\end{note}
 
+\pnum
+In the \grammarterm{decl-specifier-seq} of the \grammarterm{lambda-declarator},
+each \grammarterm{decl-specifier}
+shall either be \tcode{mutable} or \tcode{constexpr}.
+
 \pnum
 If a \grammarterm{lambda-expression} does not include a
 \grammarterm{lambda-declarator}, it is as if the \grammarterm{lambda-declarator} were
@@ -726,9 +731,6 @@
 \end{example}
 
 \pnum
-In the \grammarterm{decl-specifier-seq} of the \grammarterm{lambda-declarator},
-each \grammarterm{decl-specifier}
-shall either be \tcode{mutable} or \tcode{constexpr}.
 The function call operator or operator template is declared
 \tcode{const}~(\ref{class.mfct.non-static}) if and only if the
 \grammarterm{lambda-expression}'s \grammarterm{parameter-declaration-clause} is not