Make `behaviour_info` more accessible in documentation

[whatyouhide]

Right now, if you want to get information about the behaviour_info/1 function that gets generated in behaviours, you have to do one of the following:

• Go to the docs for the Behaviour module. This is bad as that module is deprecated.
• Google behaviour_info elixir, which returns sucky results (screenshot below)
• Google behaviour_info erlang, where for me the third result is the Modules Erlang docs, where then I have to search behaviour_info in the page.

This is all not ideal IMO, can we make it better? I’m a bit out of ideas. We could implement a mock Kernel.behaviour_info/1 function just to document stuff, but I’m not sure that's a good solution. Thoughts?

[josevalim]

We should document it in Module. That’s where we document @impl, @behaviour, and others. The Behavior module can point to the new docs we add.

[whatyouhide]

@josevalim yeah that's gonna be a start. Currently searching for @impl in Hexdocs is not a good experience but something turns up at least 😉 But that's an ex_doc thing and I'll open an issue in there. I'll add docs to Module and link from Behaviour.