Patch #809535: Mention behaviour of seek on text files. Backported to 2.3.

Doc/lib/libstdtypes.tex

@@ -1337,7 +1337,12 @@ flush the read-ahead buffer.
   \method{seek()} operations will be undone at the next write.  If the
   file is only opened for writing in append mode (mode \code{'a'}),
   this method is essentially a no-op, but it remains useful for files
-  opened in append mode with reading enabled (mode \code{'a+'}).
+  opened in append mode with reading enabled (mode \code{'a+'}).  If the
+  file is opened in text mode (mode \code{'t'}), only offsets returned
+  by \method{tell()} are legal.  Use of other offsets causes undefined
+  behavior.
+
+  Note that not all file objects are seekable.
 \end{methoddesc}
 
 \begin{methoddesc}[file]{tell}{}

Objects/fileobject.c

@@ -1613,7 +1613,9 @@ PyDoc_STRVAR(seek_doc,
 "0 (offset from start of file, offset should be >= 0); other values are 1\n"
 "(move relative to current position, positive or negative), and 2 (move\n"
 "relative to end of file, usually negative, although many platforms allow\n"
-"seeking beyond the end of a file).\n"
+"seeking beyond the end of a file).  If the file is opened in text mode,\n"
+"only offsets returned by tell() are legal.  Use of other offsets causes\n"
+"undefined behavior."
 "\n"
 "Note that not all file objects are seekable.");