DOC: jshell tool: /exit now allows snippet argument

See JDK-8185840 and new doc comments:

jshell> /help /exit
|
| /exit
|
| Leave the jshell tool. No work is saved.
| Save any work before using this command
|
| /exit
| Leave the jshell tool. The exit status is zero.
|
| /exit <integer-expression-snippet>
| Evaluate the snippet. If the snippet fails or is not an integer expression,
| display the error. Otherwise leave the jshell tool with the
| value of the expression as the exit status