RFR: 7903870: add info about return codes (exit codes) to man page

[Jonathan Gibbons]

Please review a small update to the `man` page to document the exit codes for the tool.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into 7903870.exit-status-docs
 - CODETOOLS-7903870: add info about return codes (exit codes) to man page

Changes: https://git.openjdk.org/apidiff/pull/27/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=27&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903870
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/apidiff/pull/27.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/27/head:pull/27

PR: https://git.openjdk.org/apidiff/pull/27