Patch #809535: Mention behaviour of seek on text files. Backported to 2.3.
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 6f90fb9..ff66117 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -1613,7 +1613,9 @@
"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.");