Mention that SoftBody's spatial attachments need to be instanced before the SoftBody node

**Your Godot version:** 3.5 beta 1

**Issue description:**
If softBody node is instanced before the mesh that it is referencing in spatial attachments, the editor throws the error:
`
scene\3d\spatial.cpp:272 - Condition "!is_inside_tree()" is true. Returned: Transform()
`

the error is only shown in the editor and everything works fine in game.

**This is a very difficult** error to track as it doesn't point to a node in the scene, and for instance in my case, i had a large scene with many objects, meaning that in order to find what the issue was I had to spend up to an hour, duplicating the project and removing nodes one by one in order to find which one was causing it.

The error above is shown when the softbody node is higher in the hierarchy then the meshinstance it references in spatial attachments, if it is placed lower as can be seen below, everything works as expected:

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/physics/soft_body.html