RFR: Improve version check, and related doc update

[Jonathan Gibbons]

Please review a change to improve the check for old versions of `tidy`, including specifically, the default version on mac OS.

I also fixed trailing whitespace.

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Commit messages:
 - fix trailing space
 - Improve version check, and related doc update

Changes: https://git.openjdk.org/doccheck/pull/4/files
 Webrev: https://webrevs.openjdk.org/?repo=doccheck&pr=4&range=00
  Stats: 60 lines in 2 files changed: 20 ins; 0 del; 40 mod
  Patch: https://git.openjdk.org/doccheck/pull/4.diff
  Fetch: git fetch https://git.openjdk.org/doccheck pull/4/head:pull/4

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