Add doc strings for clojure-json aliases

I was using cheshire from the REPL and got confused when I saw decode-stream and parse-stream as functions available. I went to get the docs on decode-stream and there was nothing there which confused me further. Looking at the source cleared it up and showed me that decode-stream was an alias to parse-stream for clojure-json users. I thought it could be helpful for other people if there were doc strings on the clojure-json aliases.