Document that degenerate lines wont be drawn

bug:7658830

Skia and the HW acceleration path don't render anything for degenerate
lines.

Change-Id: I5ea6fe8f16533e5045a7cd10475f4240f6c311b6
diff --git a/graphics/java/android/graphics/Canvas.java b/graphics/java/android/graphics/Canvas.java
index fc84715..91688db 100644
--- a/graphics/java/android/graphics/Canvas.java
+++ b/graphics/java/android/graphics/Canvas.java
@@ -867,8 +867,11 @@
 
     /**
      * Draw a line segment with the specified start and stop x,y coordinates,
-     * using the specified paint. NOTE: since a line is always "framed", the
-     * Style is ignored in the paint.
+     * using the specified paint.
+     *
+     * <p>Note that since a line is always "framed", the Style is ignored in the paint.</p>
+     *
+     * <p>Degenerate lines (length is 0) will not be drawn.</p>
      *
      * @param startX The x-coordinate of the start point of the line
      * @param startY The y-coordinate of the start point of the line