added wxStringOutputStream::TellO(); fixed bugs in OnSysWrite()

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@29226 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/sostream.tex

@@ -13,7 +13,7 @@
 
 This class implements an output stream which writes data either to a
 user-provided or internally allocated string. Note that currently this stream
-does not support seeking.
+does not support seeking but can tell its current position.
 
 \wxheading{Derived from}
 
@@ -34,6 +34,12 @@ If the provided pointer is non-\texttt{NULL}, data will be written to it.
 Otherwise, an internal string is used for the data written to this stream, use 
 \helpref{GetString()}{wxstringoutputstreamgetstring} to get access to it.
 
+If \arg{str} is used, data written to the stream is appended to the current
+contents of it, i.e. the string is not cleared here. However if it is not
+empty, the positions returned by \helpref{TellO}{wxoutputstreamtello} will be
+offset by the initial string length, i.e. initial stream position will be the
+initial length of the string and not $0$.
+
 
 \membersection{wxStringOutputStream::GetString}\label{wxstringoutputstreamgetstring}