RFR: 8333599: Improve description of \b matcher in j.u.r.Pattern

[Raffaello Giulietti]

A documentation-only change to match the original intent and the implemented behavior.

-------------

Commit messages:
 - 8333599: Improve description of \b matcher in j.u.r.Pattern

Changes: https://git.openjdk.org/jdk/pull/19583/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19583&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333599
  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19583.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19583/head:pull/19583

PR: https://git.openjdk.org/jdk/pull/19583