For Construct 2, check the Beginner's guide to Construct 2 written by Ashley, the developper of Construct 2 himself. You can also follow up with this video serie showing how to port the game 1945 in C2.
+ More tutorials and articles have been written for C2 since the first version of this article.
+ You also have a folder "examples" in the base folder of C2, covering quite a few basics (and a bit more advanced) areas of C2, its plugins/behaviors, ...
Extract from this tutorial
Concerning the "legend", it's already there.
In action/expression/condition windows, when you click an element there's an inline description explaining what is the purpose of the element you clicked.
Same when you set an action, clicking a parameter field will display a line of explanation.
(check the section "Inline help" in this tutorial with screenshots)
I strongly suggest you to take a look at the examples in the C2 folder, it is a true gold mine. You may try to tweak them (make a backup copy first) but I first suggest you to try to understand how they work, and why they are coded the way they are, what's the logic in it.
Then you might consider start your own project.