RFR: More details around @bug

[Jesper Wilhelmsson]

Minor change to clarify what kind of issues to refer to with the @bug tag.

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Commit messages:
 - More details around @bug

Changes: https://git.openjdk.org/guide/pull/96/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=96&range=00
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/guide/pull/96.diff
  Fetch: git fetch https://git.openjdk.org/guide pull/96/head:pull/96

PR: https://git.openjdk.org/guide/pull/96