Clarify further; fix type error

Author: dhalbert

shared-bindings/usb_cdc/Serial.c

@@ -63,13 +63,13 @@
 //|         :rtype: bytes"""
 //|         ...
 //|
-//|     def readline(self, size=-1) -> Optional[bytes]:
-//|         r"""Read a line, ending in a newline character ("\\n"), or
-//|         return everything readable if no newline is found and ``timeout`` is 0.
+//|     def readline(self, size: int = -1) -> Optional[bytes]:
+//|         r"""Read a line ending in a newline character ("\\n"), including the newline.
+//|         Return everything readable if no newline is found and ``timeout`` is 0.
 //|         Return ``None`` in case of error.
 //|
 //|         This is a binary stream: the newline character "\\n" cannot be changed.
-//|         If the host computer transmits "\\r" it will be included as part of the line.
+//|         If the host computer transmits "\\r" it will also be included as part of the line.
 //|
 //|         :param int size: maximum number of characters to read. ``-1`` means as many as possible.
 //|         :return: the line read