doc: update/extend cancel_read/write, mention new Posix support
diff --git a/documentation/pyserial_api.rst b/documentation/pyserial_api.rst
index e0101d1..b038f91 100644
--- a/documentation/pyserial_api.rst
+++ b/documentation/pyserial_api.rst
@@ -523,17 +523,29 @@
.. method:: cancel_read()
+ :platform: Posix
:platform: Windows
- Cancel a pending read operation from an other thread.
+ Cancel a pending read operation from an other thread. A blocking
+ :meth:`read` call is aborted immediately. :meth:`read` will not report
+ any error but return all data received up to that point (similar to a
+ timeout).
+
+ On Posix a call to `cancel_read()` may cancel a future :meth:`read` call.
.. versionadded:: 3.1
.. method:: cancel_write()
+ :platform: Posix
:platform: Windows
- Cancel a pending write operation from an other thread.
+ Cancel a pending write operation from an other thread. The
+ :meth:`write` method will return immediately (no error indicated).
+ However the OS may still be sending from the buffer, a separate call to
+ :meth:`reset_output_buffer` may be needed.
+
+ On Posix a call to `cancel_write()` may cancel a future :meth:`write` call.
.. versionadded:: 3.1