Mention that the not operator cannot be used with Strings and StringNames

doc/classes/String.xml

@@ -6,7 +6,7 @@
 	<description>
 		This is the built-in string Variant type (and the one used by GDScript). Strings may contain any number of Unicode characters, and expose methods useful for manipulating and generating strings. Strings are reference-counted and use a copy-on-write approach (every modification to a string returns a new [String]), so passing them around is cheap in resources.
 		Some string methods have corresponding variations. Variations suffixed with [code]n[/code] ([method countn], [method findn], [method replacen], etc.) are [b]case-insensitive[/b] (they make no distinction between uppercase and lowercase letters). Method variations prefixed with [code]r[/code] ([method rfind], [method rsplit], etc.) are reversed, and start from the end of the string, instead of the beginning.
-		[b]Note:[/b] In a boolean context, a string will evaluate to [code]false[/code] if it is empty ([code]""[/code]). Otherwise, a string will always evaluate to [code]true[/code].
+		[b]Note:[/b] In a boolean context, a string will evaluate to [code]false[/code] if it is empty ([code]""[/code]). Otherwise, a string will always evaluate to [code]true[/code]. The [code]not[/code] operator cannot be used. Instead, [method is_empty] should be used to check for empty strings.
 	</description>
 	<tutorials>
 		<link title="GDScript format strings">$DOCS_URL/tutorials/scripting/gdscript/gdscript_format_string.html</link>

doc/classes/StringName.xml

@@ -8,7 +8,7 @@
 		You will usually just pass a [String] to methods expecting a [StringName] and it will be automatically converted, but you may occasionally want to construct a [StringName] ahead of time with the [StringName] constructor or, in GDScript, the literal syntax [code]&"example"[/code].
 		See also [NodePath], which is a similar concept specifically designed to store pre-parsed scene tree paths.
 		All of [String]'s methods are available in this class too. They convert the [StringName] into a string, and they also return a string. This is highly inefficient and should only be used if the string is desired.
-		[b]Note:[/b] In a boolean context, a [StringName] will evaluate to [code]false[/code] if it is empty ([code]StringName("")[/code]). Otherwise, a [StringName] will always evaluate to [code]true[/code].
+		[b]Note:[/b] In a boolean context, a [StringName] will evaluate to [code]false[/code] if it is empty ([code]StringName("")[/code]). Otherwise, a [StringName] will always evaluate to [code]true[/code]. The [code]not[/code] operator cannot be used. Instead, [method is_empty] should be used to check for empty [StringName]s.
 	</description>
 	<tutorials>
 	</tutorials>