Add a summary comment to blink::Supplementable.

There is existing documentation about the details, but as yet no overall
"what is this thing" summary.

Change-Id: I057d1c67a8f7d5fe1cbbeb65505a3be0419bfafc
Reviewed-on: https://chromium-review.googlesource.com/1246951
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594696}

third_party/blink/renderer/platform/supplementable.h

@@ -40,6 +40,16 @@ namespace blink {
 
 // What you should know about Supplementable and Supplement
 // ========================================================
+// Supplementable allows a garbage-collected object to be extended with
+// additional data.
+//
+// Most commonly, this is used to attach data to a central object, such as
+// LocalFrame, so that it can be easily accessed. This is similar to adding a
+// member to that class (e.g. it is kept alive while the supplementable is),
+// except that it occupies less memory if not used, and can be done in cases
+// that would otherwise be a layering violation. For example, it is common for
+// features implemented in modules/ to supplement classes in core/.
+//
 // Supplementable and Supplement instances are meant to be thread local. They
 // should only be accessed from within the thread that created them. The
 // 2 classes are not designed for safe access from another thread. Violating