Bug 854296, part 2 - Document Moz2D's JoinStyle::MITER to note that its behavior is backend dependant. r=Bas

gfx/2d/Types.h

@@ -206,8 +206,10 @@ MOZ_END_ENUM_CLASS(PatternType)
 MOZ_BEGIN_ENUM_CLASS(JoinStyle, int8_t)
   BEVEL,
   ROUND,
-  MITER,
-  MITER_OR_BEVEL
+  MITER, //!< Mitered if within the miter limit, else, if the backed supports
+         //!< it (D2D), the miter is clamped. If the backend does not support
+         //!< miter clamping the behavior is as for MITER_OR_BEVEL.
+  MITER_OR_BEVEL //!< Mitered if within the miter limit, else beveled.
 MOZ_END_ENUM_CLASS(JoinStyle)
 
 MOZ_BEGIN_ENUM_CLASS(CapStyle, int8_t)