RFR: 8297194: Add a @sealedGraph tag to Buffer

[Per Minborg]

This PR proposes to opt in for graphic rendering of the sealed hierarchy of the `Buffer` class.

Rendering capability was added via https://bugs.openjdk.org/browse/JDK-8295653

Here is how it would look like:

<img width="475" alt="Buffer_SH" src="https://user-images.githubusercontent.com/7457876/202420742-4d379512-1d8c-4b32-8d17-d2e70faa3a8f.png">

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Commit messages:
 - Add a @sealedGraph tag to Reference

Changes: https://git.openjdk.org/jdk/pull/11206/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11206&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297194
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11206.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11206/head:pull/11206

PR: https://git.openjdk.org/jdk/pull/11206