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

[Ian Graves]

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

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Commit messages:
 - Updating documentation on comments mode and character class whitespace

Changes: https://git.openjdk.java.net/jdk/pull/4618/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4618&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8199594
  Stats: 12 lines in 1 file changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4618.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4618/head:pull/4618

PR: https://git.openjdk.java.net/jdk/pull/4618