docs/reference/speed_python: Update native emitter limitations.

This commit updates the listed limitations of the native emitter in the
documentation related to how to increase speed of python code.

Context managers are supported, as in functions marked as native can use
the `with` statement in regular code.  Generators can be used in native
functions both on the emitting (ie. `yield <whatever>`) and on the
receiving end.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti

docs/reference/speed_python.rst

@@ -243,12 +243,10 @@ no adaptation (but see below). It is invoked by means of a function decorator:
 
 There are certain limitations in the current implementation of the native code emitter.
 
-* Context managers are not supported (the ``with`` statement).
-* Generators are not supported.
 * If ``raise`` is used an argument must be supplied.
 * The background scheduler (see `micropython.schedule`) is not run during
   execution of native code.
-* On targets with thrteading and the GIL, the GIL is not released during
+* On targets with threading and the GIL, the GIL is not released during
   execution of native code.
 
 To mitigate the last two points, long running native functions should call