[depr.str.strstreams] Add synopsis for <strstream> header (#1922)

Author: jensmaurer

source/future.tex

@@ -293,13 +293,24 @@
 
 \rSec1[depr.str.strstreams]{\tcode{char*} streams}
 
+\rSec2[depr.strstream.syn]{Header \tcode{<strstream>} synopsis}
+
 \pnum
 The header
 \indexhdr{strstream}%
 \tcode{<strstream>}
-defines three types that associate stream buffers with
+defines types that associate stream buffers with
 character array objects and assist reading and writing such objects.
 
+\begin{codeblock}
+namespace std {
+  class strstreambuf;
+  class istrstream;
+  class ostrstream;
+  class strstream;
+}
+\end{codeblock}
+
 \rSec2[depr.strstreambuf]{Class \tcode{strstreambuf}}
 
 \indexlibrary{\idxcode{strstreambuf}}%