Add missing truthiness documentation

doc/classes/PackedInt64Array.xml

@@ -8,6 +8,7 @@
 		[b]Note:[/b] This type stores signed 64-bit integers, which means it can take values in the interval [code][-2^63, 2^63 - 1][/code], i.e. [code][-9223372036854775808, 9223372036854775807][/code]. Exceeding those bounds will wrap around. If you only need to pack 32-bit integers tightly, see [PackedInt32Array] 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. [PackedInt64Array] versus [code]Array[int][/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. 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>