update documentation for Twitter::REST::Tweets#update
update documentation for Twitter::REST::Tweets#update

per discussion on #656, fallback behavior wasn't documented

megashurik/twitter,stefansundin/twitter,mike-north/twitter,creeshla/ruby-twitter,thomas-holmes/twitter,roschaefer/twitter,dantame/twitter,stkn417/twitter,paulgonz6/twitter,develon/twitter,ariarijp/twitter,SophieDeBenedetto/twitter,kevinmel2000/twitter,kmcmcurtis7/twitter,DarbySmart/twitter