RFR: 8199594: Add doc describing how (?x) ignores spaces in character classes [v2]

[Ian Graves]

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

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

  delete errant line

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4618/files
  - new: https://git.openjdk.java.net/jdk/pull/4618/files/07ea6f0f..11bafe80

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4618&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4618&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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