RFR: 8265612: revise the help info for jmap histo command [v3]

[Chris Plummer]

On Thu, 22 Apr 2021 05:53:35 GMT, Lin Zang <lzang at openjdk.org> wrote:

>> This PR  revise the help message of the `parallel=[N]` option of command `jmap -histo`. It mainly comes from the discussion at  https://github.com/openjdk/jdk/pull/2379#issuecomment-782609582  and https://github.com/openjdk/jdk/pull/2261#issuecomment-823623186.
>
> Lin Zang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix indentation issue

Marked as reviewed by cjplummer (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/3598