[format.parse.ctx] Move non-normative explanations of errors into notes (#6816)

Author: Eisenwave

source/utilities.tex

@@ -17353,8 +17353,10 @@
 
 \pnum
 \throws
-\tcode{format_error} if \tcode{indexing_ == manual} is \tcode{true}
-which indicates mixing of automatic and manual argument indexing.
+\tcode{format_error} if \tcode{indexing_ == manual} is \tcode{true}.
+\begin{note}
+This indicates mixing of automatic and manual argument indexing.
+\end{note}
 
 \pnum
 \remarks
@@ -17380,8 +17382,10 @@
 \pnum
 \throws
 \tcode{format_error} if
-\tcode{indexing_ == automatic} is \tcode{true}
-which indicates mixing of automatic and manual argument indexing.
+\tcode{indexing_ == automatic} is \tcode{true}.
+\begin{note}
+This indicates mixing of automatic and manual argument indexing.
+\end{note}
 
 \pnum
 \remarks