Reference Range signals in the Slider class documentations

See https://github.com/godotengine/godot-docs/issues/3837.

(cherry picked from commit eaa52cbb36)

doc/classes/HSlider.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		Horizontal slider. See [Slider]. This one goes from left (min) to right (max).
+		[b]Note:[/b] The [signal Range.changed] and [signal Range.value_changed] signals are part of the [Range] class which this class inherits from.
 	</description>
 	<tutorials>
 	</tutorials>

doc/classes/Slider.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		Base class for GUI sliders.
+		[b]Note:[/b] The [signal Range.changed] and [signal Range.value_changed] signals are part of the [Range] class which this class inherits from.
 	</description>
 	<tutorials>
 	</tutorials>

doc/classes/VSlider.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		Vertical slider. See [Slider]. This one goes from bottom (min) to top (max).
+		[b]Note:[/b] The [signal Range.changed] and [signal Range.value_changed] signals are part of the [Range] class which this class inherits from.
 	</description>
 	<tutorials>
 	</tutorials>