improve docs

including `writeString()`

doc/jsoncpp.dox

@@ -85,9 +85,8 @@ Json::StreamWriter::Builder builder;
 builder.withIndentation("   ");  // or whatever you like
 
 // Then build a StreamWriter.
-// (Of course, you can write to std::ostringstream if you prefer.)
 std::shared_ptr<Json::StreamWriter> writer(
-    builder.newStreamWriter( &std::cout );
+    builder.newStreamWriter( &std::cout ) );
 
 // Make a new JSON document for the configuration. Preserve original comments.
 writer->write( root );
@@ -95,6 +94,10 @@ writer->write( root );
 // If you like the defaults, you can insert directly into a stream.
 std::cout << root;
 
+// Of course, you can write to `std::ostringstream` if you prefer. Or
+// use `writeString()` for convenience.
+std::string document = Json::writeString( root, builder );
+
 // You can also read from a stream.  This will put the contents of any JSON
 // stream at a particular sub-value, if you'd like.
 std::cin >> root["subtree"];