Android Clipboard: Update legacy implementation comment.

Update the comment and explain it a bit better. Bitmaps have been
supported in the Native Android clipboard since https://crrev.com/c/2147890.

While I'm unsure of the first instance of this comment, an early
iteration is active in March 2012(https://crrev.com/9264014),
and a recent, more well-documented change that expanded this comment
is available here(https://crrev.com/12633009)

Change-Id: I192d3abd050577c026f126ed99c9d9313756555c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382881
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802884}

ui/base/clipboard/clipboard_android.cc

@@ -32,16 +32,21 @@
 #include "ui/gfx/android/java_bitmap.h"
 #include "ui/gfx/image/image.h"
 
-// TODO:(andrewhayden) Support additional formats in Android: Bitmap, URI, HTML,
+// TODO:(andrewhayden) Support additional formats in Android: URI, HTML,
 // HTML+text now that Android's clipboard system supports them, then nuke the
 // legacy implementation note below.
 
 // Legacy implementation note:
-// The Android clipboard system used to only support text format. So we used the
-// Android system when some text was added or retrieved from the system. For
-// anything else, we STILL store the value in some process wide static
-// variable protected by a lock. So the (non-text) clipboard will only work
-// within the same process.
+// The Android clipboard, and hence `ClipboardAndroid` as well, used to only
+// support text. Since then, bitmap support has been added, but not support
+// for any other formats.
+//
+// Therefore, Clipboard data is stored:
+// - on the Android system, for text and bitmaps.
+// - in a process-wide static variable protected by a lock, for other formats.
+//
+// These "other formats" only work within the same process, and can't be copied
+// between Android applications.
 
 using base::android::AttachCurrentThread;
 using base::android::ClearException;