Rework if/2 docs and mention variable scope

[sabiwara]

The structure of the if doc was a bit hard to understand, since:

• we now had the following 3 sections: "One-liner examples" / "Blocks examples" / "Examples"
• some examples were far away from the points they were illustrating (e.g. "Otherwise, nil will be
  returned.")
• mixing of the if(foo, do: bar) and if foo, do: bar form, but AFAIK the former is less common and idiomatic nowadays?

I also noticed that the fact we can define a variable in the if condition wasn't explicitly documented, as well as scope rules more broadly (even if they aren't specific to if, it's probably the more common place people encounter it). Inspired from this

[dmitrykleymenov]

In the example above, "yes" is returned because 7 > 5 evaluates to a truthy value (true in this case). nil will be returned if the condition evaluates to a falsy(nil or false) value:

[sabiwara]

Thanks @dmitrykleymenov, good catch!
Fixed in 96bbccf

[dmitrykleymenov]

I'm just curious if it should be mentioned that the code inside unmatched blocks isn't even evaluated, and it's not only about a return value.

[sabiwara]

I'm just curious if it should be mentioned that the code inside unmatched blocks isn't even evaluated, and it's not only about a return value.

It's true that it wasn't explicitly called out, added an example in 6d56287