docs/library/uasyncio.rst: Document stream readexactly() method.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/uasyncio.rst

@@ -248,6 +248,14 @@ TCP stream connections
 
     This is a coroutine, and a MicroPython extension.
 
+.. method:: Stream.readexactly(n)
+
+    Read exactly *n* bytes and return them as a bytes object.
+
+    Raises an ``EOFError`` exception if the stream ends before reading *n* bytes.
+
+    This is a coroutine.
+
 .. method:: Stream.readline()
 
     Read a line and return it.