GDScript: Clarify the use of callv() to call variadic functions

[dalexeev]

• Closes Empty variadic parameters of functions called in cascade will accumulate empty array elements godot#112832.

[AThousandShips]

LGTM!

[mhilbrunner]

Content as is looks good to me. Should this really be in the comment instead of an explanation outside the code block, thinking about translations and such?

[MikeSchulze]

What's about constructors using variadic arguments?

func foo(...args: Array) -> Foo:
	return Foo.new(args)
	
class Foo:
   func _init(...args: Array) -> void:
	_values = args	

I use this hack to workaround

func _init(...args: Array) -> void:
	if args.size() == 1 and args[0] is Array:
		_values = args[0]
		return
	_values = args

It would be great instead to write special documentation to support variadic argument delegation over function borders.