Document which Cache methods are optional

[gissuebot]

Original issue created by lowasser@google.com on 2013-03-01 at 06:50 PM

---

I'm fairly certain there are some methods I would've expected to be obligatory but aren't implemented everywhere?

[gissuebot]

Original comment posted by kevinb@google.com on 2013-03-12 at 06:43 PM

---

(No comment entered for this change.)

---

CC: fry@google.com

[cgdecker]

This issue is quite old, but it's not clear what methods this issue is referring to or what the issue is. Some methods on Cache throw UOE in some places, or something?