Update PackedVector4Array description to include explanation shared by PackedArray classes

doc/classes/PackedVector4Array.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		An array specifically designed to hold [Vector4]. Packs data tightly, so it saves memory for large array sizes.
+		[b]Differences between packed arrays, typed arrays, and untyped arrays:[/b] Packed arrays are generally faster to iterate on and modify compared to a typed array of the same type (e.g. [PackedVector4Array] versus [code]Array[Vector4][/code]). Also, packed arrays consume less memory. As a downside, packed arrays are less flexible as they don't offer as many convenience methods such as [method Array.map]. Typed arrays are in turn faster to iterate on and modify than untyped arrays.
 		[b]Note:[/b] Packed arrays are always passed by reference. To get a copy of an array that can be modified independently of the original array, use [method duplicate]. This is [i]not[/i] the case for built-in properties and methods. The returned packed array of these are a copies, and changing it will [i]not[/i] affect the original value. To update a built-in property you need to modify the returned array, and then assign it to the property again.
 	</description>
 	<tutorials>