Function yield better not used inside _ready function

**Your Godot version:**
2.1 it might applies to v3.x since the yielding function and the node source hasn't changed the yielding behavior

**Issue description:**
Using `yield` inside the `_ready` function might lead to an unexpected behavior that's really hard to reproduce.
I'm not sure, but it seems that using `yield` inside the `_ready` function might lead to a node duplication, it is quite rare and random, and really really hard to reproduce.

Also, while browsing old github issues, I found out [that comment](https://github.com/godotengine/godot/issues/5247#issuecomment-226645193), and I think it'd be better to add a note to the documentation of the yielding function explaining it's better to avoid yielding inside the `_ready` function.

**URL to the documentation page:**
https://docs.godotengine.org/fr/stable/classes/class_@gdscript.html#class-gdscript-method-yield