Added description to is_pixel_opaque() in Sprite.xml

(cherry picked from commit 44a20a0f61)

doc/classes/Sprite.xml

@@ -28,6 +28,8 @@
 			<argument index="0" name="pos" type="Vector2">
 			</argument>
 			<description>
+				Returns [code]true[/code], if the pixel at the given position is opaque and [code]false[/code] in other case.
+				Note: It also returns [code]false[/code], if the sprite's texture is null or if the given position is invalid.
 			</description>
 		</method>
 	</methods>