RFR: 7903802: apidiff: minor doc and related updates

[Jonathan Gibbons]

Please review some minor doc changes for apidiff

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Commit messages:
 - 7903802: apidiff: minor doc and related updates

Changes: https://git.openjdk.org/apidiff/pull/4/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=4&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903802
  Stats: 18 lines in 2 files changed: 2 ins; 4 del; 12 mod
  Patch: https://git.openjdk.org/apidiff/pull/4.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/4/head:pull/4

PR: https://git.openjdk.org/apidiff/pull/4