RFR: 8199594: Add doc describing how (?x) ignores spaces in character classes

[Ian Graves]

Clarifying note on comments mode to explicitly note that whitespace within character classes is ignored.

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Commit messages:
 - Note about comments mode and character class whitespace

Changes: https://git.openjdk.java.net/jdk/pull/3577/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3577&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8199594
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3577.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3577/head:pull/3577

PR: https://git.openjdk.java.net/jdk/pull/3577