Document the upsides and downsides of packed arrays versus typed arrays

doc/classes/PackedFloat64Array.xml

@@ -6,6 +6,7 @@
 	<description>
 		An array specifically designed to hold 64-bit floating-point values (double). Packs data tightly, so it saves memory for large array sizes.
 		If you only need to pack 32-bit floats tightly, see [PackedFloat32Array] for a more memory-friendly alternative.
+		[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. [PackedFloat64Array] versus [code]Array[float][/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>
 	</tutorials>