Your Godot version: 4.4

Issue description:

Related to #368 .

There is currently no documentation for GDScript warnings in the Godot manual. I believe it may be helpful to create pages describing each warning in more detail, along with examples of code that causes them and an explanation of how to fix them.

Microsoft's official C# documentation has pages describing exceptions that can be thrown, with similar sections: https://learn.microsoft.com/en-us/dotnet/api/system.nullreferenceexception?view=net-9.0

For reference, here is my shot at writing a page for the GET_NODE_DEFAULT_WITHOUT_ONREADY warning:

GET_NODE_DEFAULT_WITHOUT_ONREADY

The warning that appears when a node's default value is set to a location in the scene tree without using the @onready annotation.
The message will say something like

The default value is using "$" which won't return nodes in the scene tree before "_ready()" is called. Use the "@onready" annotation to solve this.

When this warning occurs

In GDScript, instance variables can be set by declaring them outside of a method. Additionally, they can be given a default value using =:

extends Area2D
var my_num = 3

This way, the variable my_num will always start out with a value of 3 in each instance of this class.
GDScript also has methods to retrieve specific nodes from the scene tree: namely, the get_node method, and its shorthand versions $ (and % for unique nodes). Thus, if you want to have an instance variable default to a child of the node with a script, it may be tempting to write something like the following:

extends Area2D
var my_collision_shape = $CollisionShape2D

However, class instance variables' default values are evaluated and assigned before the scene tree is set up. This means that at the time of assigning the default value, the node may not be in the scene tree, and the variable will be set to null instead.

How to fix this warning

The most straightforward solution is to add the @onready annotation before your variable declaration:

extends Area2D
@onready var my_collision_shape = $CollisionShape2D

Now, the default value of the variable will not be assigned until the scene tree has been initialized, and the target node will be present.

Alternatively, you can set the value of the variable at the beginning of your _ready method:

extends Area2D
var my_collision_shape

func _ready():
    my_collision_shape = $CollisionShape2D

While I tried to keep everything accurate to the best of my knowledge, there might be some inaccuracies in this write-up. Of course, if this proposal were to move forward, we would want the content of the pages to be reviewed by multiple people before going live.
Hopefully, though, this example conveys the spirit of what I'm proposing.

The editor could potentially even provide a link to the corresponding page when a user encounters a particular warning, so that the user can read more about it.