[stringbuf.members] Remove // exposition only from itemdecl (#8179)

Author: frederick-vs-ja

source/iostreams.tex

@@ -8416,7 +8416,7 @@
 (except for those characters re-initialized by the new \tcode{basic_string}).
 
 \begin{itemdecl}
-void @\exposid{init-buf-ptrs}@(); // \expos
+void @\exposid{init-buf-ptrs}@();
 \end{itemdecl}
 
 \begin{itemdescr}