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

[Ian Graves]

On Wed, 21 Apr 2021 02:56:12 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> Ian Graves has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Adding differences to Perl 5 note
>
> A few minor wording adjustments. Please update the CSR accordingly and I'll review it too.

Wording updated per @stuart-marks's suggestions. Thanks! CSR updated accordingly.

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