RFR: 8331018: Clean up non-standard use of /** comments in `jdk.jshell`

[Jonathan Gibbons]

Please review a fix to preempt warnings that would otherwise be introduced by [JDK-8303689](https://bugs.openjdk.org/browse/JDK-8303689), which introduces a new javac lint warning to detect documentation comments in unusual places.

The comments here are all one-line comments of the form `/***** message ****/`.

Following the style of an existing similar comment, the comments are changed to the form `//***** message *****`
That existing comment is in Snippet.java:648

    //**** internal access ****
    String name() {
        return unitName;
    }

-------------

Commit messages:
 - JDK-8331018: Clean up non-standard use of /** comments in `jdk.jshell`

Changes: https://git.openjdk.org/jdk/pull/18926/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18926&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331018
  Stats: 7 lines in 7 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/18926.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18926/head:pull/18926

PR: https://git.openjdk.org/jdk/pull/18926