Bug 1549267 Part 2 - Improve comments in LayoutConstants.h. r=mats

Differential Revision: https://phabricator.services.mozilla.com/D31697

--HG--
extra : moz-landing-system : lando

layout/base/LayoutConstants.h

@@ -13,11 +13,14 @@
 
 /**
  * Constant used to indicate an unconstrained size.
+ *
+ * NOTE: The constants defined in this file are semantically used as symbolic
+ *       values, so user should not depend on the underlying numeric values. If
+ *       new specific use cases arise, define a new constant here.
  */
 #define NS_UNCONSTRAINEDSIZE NS_MAXSIZE
 
-// NOTE: There are assumptions all over that these have the same value,
-//       namely NS_UNCONSTRAINEDSIZE.
+// NS_AUTOOFFSET is assumed to have the same value as NS_UNCONSTRAINEDSIZE.
 #define NS_AUTOOFFSET NS_UNCONSTRAINEDSIZE
 
 // +1 is to avoid clamped huge margin values being processed as auto margins