Document the upsides and downsides of packed arrays versus typed arrays

doc/classes/PackedStringArray.xml

@@ -11,6 +11,7 @@
 		var string = " ".join(string_array)
 		print(string) # "hello world"
 		[/codeblock]
+		[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. [PackedStringArray] versus [code]Array[String][/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.
 	</description>
 	<tutorials>
 		<link title="OS Test Demo">https://godotengine.org/asset-library/asset/677</link>