[O] ":results none" doesn't seem to be documented

[Kodi Arfer]

"none" is allowed as an argument to :results (see, for example, 
ob-core.el line 704 as of Git d36bd8d), but this isn't mentioned in 
results.html. I just learned of its existence while reading ob-core.el.

To me, by the way, ":results none" seems like a useful feature rather 
than being redundant with ":results silent", because it won't attempt to 
print a massive object to the echo area. (Pehaps "silent" wasn't a great 
choice of name.)