Updating the SIL doc

[dan-zheng]

@Michael_Gottesman

background : I am not sure if you are aware but the SIL parser/printer has been a place where the Swift compiler has had technical debt for a long time that needs to be paid down. So matching the current implementation would mean I think matching whatever weirdness is inherent in it.

I'm sorry, I'm not sure how this relates to the topic of SIL documentation - maybe I missed some connections. Could you please clarify?

Maybe a halfway/orthogonal idea is: generating SIL documentation from C++ doc comments for SIL instructions.