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.
------------- 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
