RFR [9] 8068599: Apply mutability, serializability, and thread-safety, caveat to all Collectors that do not accept a Collection supplier

[Chris Hegarty]

This is a review request for a small spec clarification to java.util.stream.Collectors, that appears to be a minor oversight in the original spec.

Many of the Collectors that accumulate, group, etc, elements into a Collection, and do not accept a Collection supplier, come with a caveat on the Collection they create, along the lines of, "There are no guarantees on the type, mutability, serializability, or thread-safety of the ... returned". 

This caveat should be applied to all Collectors that create such Collections.

Specdiff:
   http://cr.openjdk.java.net/~chegar/8068599/specdiff/java/util/stream/Collectors.html
webrev:
   http://cr.openjdk.java.net/~chegar/8068599/webrev/

Thanks,
-Chris.