[range.istream.view] Add reference for basic_istream_view::iterator (#5514)

Author: hewillk

source/ranges.tex

@@ -3257,6 +3257,7 @@
     constexpr default_sentinel_t end() const noexcept;
 
   private:
+    // \ref{range.istream.iterator}, class \tcode{basic_istream_view::\exposid{iterator}}
     struct @\exposidnc{iterator}@;                            // \expos
     basic_istream<CharT, Traits>* @\exposid{stream_}@;      // \expos
     Val @\exposid{value_}@ = Val();                         // \expos