RFR: 8300587: (bf) Some covariant overrides are missing @since tags

[Brian Burkhalter]

Reinstate `@since 9` tags for `MappedByteBuffer` and `$TypeBuffer` covariant overrides.

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Commit messages:
 - 8300587: (bf) Some covariant overrides are missing @since tags

Changes: https://git.openjdk.org/jdk/pull/12123/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12123&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300587
  Stats: 16 lines in 2 files changed: 14 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12123.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12123/head:pull/12123

PR: https://git.openjdk.org/jdk/pull/12123