Improve the `AudioStreamPlayer2D/3D.area_mask` documentation

The documentation was stating that the area mask affects where sounds
can be heard, which is not true. Instead, the area mask affects audio
bus redirection.

Thanks to Azedaxen on the Godot forums for providing this description :)

(cherry picked from commit ab1eaac315acd6a763ae64902c3bb797838b60ba)

doc/classes/AudioStreamPlayer2D.xml

@@ -47,7 +47,7 @@
 	</methods>
 	<members>
 		<member name="area_mask" type="int" setter="set_area_mask" getter="get_area_mask" default="1">
-			Areas in which this sound plays.
+			Determines which [Area2D] layers affect the sound for reverb and audio bus effects. Areas can be used to redirect [AudioStream]s so that they play in a certain audio bus. An example of how you might use this is making a "water" area so that sounds played in the water are redirected through an audio bus to make them sound like they are being played underwater.
 		</member>
 		<member name="attenuation" type="float" setter="set_attenuation" getter="get_attenuation" default="1.0">
 			Dampens audio over distance with this as an exponent.

doc/classes/AudioStreamPlayer3D.xml

@@ -48,7 +48,7 @@
 	</methods>
 	<members>
 		<member name="area_mask" type="int" setter="set_area_mask" getter="get_area_mask" default="1">
-			Areas in which this sound plays.
+			Determines which [Area] layers affect the sound for reverb and audio bus effects. Areas can be used to redirect [AudioStream]s so that they play in a certain audio bus. An example of how you might use this is making a "water" area so that sounds played in the water are redirected through an audio bus to make them sound like they are being played underwater.
 		</member>
 		<member name="attenuation_filter_cutoff_hz" type="float" setter="set_attenuation_filter_cutoff_hz" getter="get_attenuation_filter_cutoff_hz" default="5000.0">
 			Dampens audio using a low-pass filter above this frequency, in Hz. To disable the dampening effect entirely, set this to [code]20500[/code] as this frequency is above the human hearing limit.