Improve description of GetNodeOrDefault in C# API

[guilleatm]

Closes godotengine/godot-docs#6577.

[guilleatm]

PR from issue in docs: godotengine/godot-docs#6577

Provided same text as GDScript version: https://docs.godotengine.org/en/latest/classes/class_node.html#class-node-method-get-node-or-null

First PR :)

[YuriSizov]

Note that you will need to squash your commits into one, with a clear but short message, before this can be merged. You can do it later, in case there are some review comments that you need to address, but you will have to do this eventually. For a reference when crafting a good commit message you can read this.

[guilleatm]

Squash done

[YuriSizov]

Thanks! And congratulations on getting your first PR merged :)

[guilleatm]

Thanks! :)