RFR: 8265612: revise the help info for jmap histo command

[Lin Zang]

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.

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Commit messages:
 - 8265612: revise the help info for jmap histo command

Changes: https://git.openjdk.java.net/jdk/pull/3598/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3598&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265612
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3598.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3598/head:pull/3598

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