bool return value of Tween functions not documented

[mitchcurtis]

None of the functions here document the meaning of their return value:

https://docs.godotengine.org/en/3.1/classes/class_tween.html#class-tween-method-interpolate-property

[rcorre]

I was about to report this in the godot-docs issue tracker, but after looking at the source, I think the issue is actually that the return value is used inconsistently and is not very useful.

Many Tween functions like start will always return true. A few will return false in the case where there's "no valid delta", but there are many other conditions where it will fail with an assert instead. So it sounds like the answer for most is that they "return true if they succeed, and maybe return false if they fail, but also might error out."

[Calinou]

I think it would make sense to make those Tween methods return void instead of a boolean value. We can still print errors if something wrong occurs.

[akien-mga]

This was fixed already by #36844.