RFR: 8199594: Add doc describing how (?x) ignores spaces in character classes [v3]

[Ian Graves]

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

Ian Graves has updated the pull request incrementally with one additional commit since the last revision:

  Tweaking wording and placement

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3577/files
  - new: https://git.openjdk.java.net/jdk/pull/3577/files/f4507e3a..79390124

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3577&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3577&range=01-02

  Stats: 11 lines in 1 file changed: 2 ins; 3 del; 6 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