RFR: 8217496: Matcher.group() can return null after usePattern

[Ian Graves]

Updates to the documentation to describe behavior in Matcher.group().

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Commit messages:
 - Updating verbiage
 - Merge branch 'master' into matcher-group-doc-update
 - 8217496 Matcher.group() can return null after usePattern

Changes: https://git.openjdk.org/jdk/pull/12835/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12835&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8217496
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12835.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12835/head:pull/12835

PR: https://git.openjdk.org/jdk/pull/12835