Add missing truthiness documentation

doc/classes/PackedByteArray.xml

@@ -7,6 +7,7 @@
 		An array specifically designed to hold bytes. Packs data tightly, so it saves memory for large array sizes.
 		[PackedByteArray] also provides methods to encode/decode various types to/from bytes. The way values are encoded is an implementation detail and shouldn't be relied upon when interacting with external apps.
 		[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. In these cases the returned packed array is a copy, and changing it will [i]not[/i] affect the original value. To update a built-in property of this type, modify the returned array and then assign it to the property again.
+		[b]Note:[/b] In a boolean context, a packed array will evaluate to [code]false[/code] if it's empty. Otherwise, a packed array will always evaluate to [code]true[/code].
 	</description>
 	<tutorials>
 	</tutorials>