[Runtime] Comment the MetadataAllocatorTags enum.

Author: mikeash

include/swift/Runtime/Metadata.h

@@ -25,6 +25,13 @@ namespace swift {
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wreturn-type-c-linkage"
 
+// Tags used to denote different kinds of allocations made with the metadata
+// allocator. This is encoded in a header on each allocation when metadata
+// iteration is enabled, and allows tools to know where each allocation came
+// from.
+//
+// Some of these values are also declared in SwiftRemoteMirrorTypes.h. Those
+// values must be kept stable to preserve compatibility.
 enum MetadataAllocatorTags : uint16_t {
   UnusedTag = 0,
   BoxesTag,