Fix old documentation about default blocking behavior of kill, killall

In version v0.13.0 this was changed, says the docstrings, but the
function signature in the rst was never updated, suggesting that the
default block behavior is "False".

doc/gevent.rst

@@ -37,7 +37,7 @@ It also a good idea to override :meth:`__str__`: if :meth:`_run` raises an excep
 .. automethod:: Greenlet.start_later
 .. automethod:: Greenlet.join
 .. automethod:: Greenlet.get
-.. automethod:: Greenlet.kill(exception=GreenletExit, block=False, timeout=None)
+.. automethod:: Greenlet.kill(exception=GreenletExit, block=True, timeout=None)
 .. automethod:: Greenlet.link(callback)
 .. automethod:: Greenlet.link_value(callback)
 .. automethod:: Greenlet.link_exception(callback)
@@ -89,7 +89,7 @@ Useful general functions
 
 .. autofunction:: kill(greenlet, exception=GreenletExit)
 
-.. autofunction:: killall(greenlets, exception=GreenletExit, block=False, timeout=None)
+.. autofunction:: killall(greenlets, exception=GreenletExit, block=True, timeout=None)
 
 .. autofunction:: joinall