[format.parse.ctx] Improve readability of paragraphs 12 and 14 (#6815)

Author: eisenwave

source/utilities.tex

@@ -17385,8 +17385,8 @@
 
 \pnum
 \remarks
-Call expressions where \tcode{id >= num_args_} is \tcode{true} are not
-core constant expressions\iref{expr.const}.
+A call to this function is a core constant expression\iref{expr.const} only if
+\tcode{id < num_args_} is \tcode{true}.
 \end{itemdescr}
 
 \indexlibrarymember{check_dynamic_spec}{basic_format_parse_context}%
@@ -17415,12 +17415,14 @@
 
 \pnum
 \remarks
-Call expressions where
-\tcode{id >= num_args_} or
+A call to this function is a core constant expression only if
+\begin{itemize}
+\item
+\tcode{id < num_args_} is \tcode{true} and
+\item
 the type of the corresponding format argument
-(after conversion to \tcode{basic_format_arg<Context>})
-is not one of the types in \tcode{Ts...}
-are not core constant expressions\iref{expr.const}.
+(after conversion to \tcode{basic_format_arg<Context>}) is one of the types in \tcode{Ts...}.
+\end{itemize}
 \end{itemdescr}
 
 \indexlibrarymember{check_dynamic_spec_integral}{basic_format_parse_context}%