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

[Brian Burkhalter]

On Mon, 19 Apr 2021 20:43:26 GMT, Ian Graves <igraves at openjdk.org> wrote:

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

Looks all right. Probably a CSR is in order.

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Marked as reviewed by bpb (Reviewer).

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