Add warning to Object._iter_init docs about using alternative state

[aurpine]

This follows the update in godotengine/godot-docs#11901 that will remove the technique referenced in Object._iter_init docs. It is replaced with a warning about not using the iter parameter.

[dalexeev]

Perhaps it would be better not just to remove the mention of the alternative, but to add a note explaining why you shouldn't use this approach (nested loops)? Also, would it be worth keeping a link to the online documentation, in case it explains the iterator API in a bit more detail?

[aurpine]

Added a section explaining the alternative. Maybe we could move this to the tutorial page instead to keep the class reference lean?

[dalexeev]

Please squash commits into one. To do this you need a local git client, GitHub web interface doesn't support it.

It might also make sense to update the commit message and PR title, given that you're no longer removing the mention, but emphasizing that this is a less desirable approach.