[range.split.inner] Clarify paragraph 1 (#4545)

... to avoid confusion about what it means for a name that is not present to denote a type.

Author: CaseyCarter

source/ranges.tex

@@ -5866,7 +5866,9 @@
 \end{codeblock}
 
 \pnum
-The \grammarterm{typedef-name} \tcode{iterator_category} denotes:
+If \exposid{Base} does not model \libconcept{forward_range}
+there is no member \tcode{iterator_category}.
+Otherwise, the \grammarterm{typedef-name} \tcode{iterator_category} denotes:
 \begin{itemize}
 \item
 \tcode{forward_iterator_tag} if