RFR: 8303130: Document required Accessibility permissions on macOS

[Dmitry Markov]

Added the section devoted to client UI test which use Robot API

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Commit messages:
 - 8303130: Document required Accessibility permissions on macOS

Changes: https://git.openjdk.org/jdk/pull/12772/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12772&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303130
  Stats: 52 lines in 2 files changed: 46 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12772.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12772/head:pull/12772

PR: https://git.openjdk.org/jdk/pull/12772