docs: add notes for readline, readlines, writelines, IOBase

see #285
diff --git a/serial/serialposix.py b/serial/serialposix.py
index 9eec8a7..507e2fe 100644
--- a/serial/serialposix.py
+++ b/serial/serialposix.py
@@ -789,6 +789,9 @@
     the error handling is degraded.
 
     Overall timeout is disabled when inter-character timeout is used.
+
+    Note that this implementation does NOT support cancel_read(), it will
+    just ignore that.
     """
 
     def _reconfigure_port(self, force_update=True):