[meta.syn] Add missing paragraph 4.

This was omitted in the application of P2996R13 by accident.

Author: tkoeppe

source/meta.tex

@@ -3338,6 +3338,11 @@
 \end{codeblock}
 \end{example}
 
+\pnum
+For the purpose of exposition,
+throughout this clause \tcode{\caret\caret\placeholder{E}} is used
+to indicate a reflection representing source construct \tcode{\placeholder{E}}.
+
 \rSec2[meta.string.literal]{Checking string literals}
 
 \indexlibraryglobal{is_string_literal}%