I've attached a patch for what I think are a few improvements to 
dictionary.el's documentation.

There are still a handful of undocumented functions/commands.
The commentary section also redirects to an external source, which 
is not very helpful:

;; You can find more information in the README file of the GitHub
;; repository https://github.com/myrkr/dictionary-el

I bumped into all this when trying out dictionary-tooltip-mode.
I thought "oh, cool", enabled the mode, then...nothing happened.
That's due to the dictionary-tooltip-dictionary defaulting to nil. 
Perhaps this user option could default to 
dictionary-default-dictionary instead.