busio/UART: Correct and clarify readline() return.

Surely readline() "rtype" is string not int as stated (and not bytes as some might expect).

Also it is not totally unambiguous what happens on a timeout so it would help to clarify in docs that on a timeout 
it does NOT return with what it has read so far, rather it leaves all that in the buffer ready for a future read and returns nothing.

Likewise clarify that if timeout=0 but there is no newline it DOES return what it has read so far (NOT None). 

At least this is what I think it does and/or is supposed to do!

Python docs are generally not too explicit about what is the proper treatment, so perhaps all the more reason to
clarify the interpretation adopted?

Author: AndrewR-L

shared-bindings/busio/UART.c

@@ -201,13 +201,15 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(busio_uart___exit___obj, 4, 4, busio_
 //|     :rtype: int or None (on a non-blocking error)
 //|
 //|     *New in CircuitPython 4.0:* No length parameter is permitted.
-
+//|
 //|   .. method:: readline()
 //|
-//|     Read a line, ending in a newline character.
+//|     Read a line, ending in a newline character, or
+//|     return None if a timeout occurs sooner, or
+//|     return everything readable if no newline is found and timeout=0 
 //|
 //|     :return: the line read
-//|     :rtype: int or None
+//|     :rtype: str or None
 //|
 //|   .. method:: write(buf)
 //|