RFR (S) 8130450, 8139835: JShell: repeated source / Snippet.id() doc

[Robert Field]

Bugs:
     8130450: JShell: events are not generated for repeated source
     8139835: JShell API: Snippet.id() doc -- specify: no meaning, dynamic

Webrev:
     http://cr.openjdk.java.net/~rfield/8130450.webrev/

Thanks,
Robert