Document cache.h interfaces based on clockcache comments.

[jonhnet]

I tried to document all the public methods on the cache.h polymorphic interface. Please see XXXs for places where I don't even have a guess. But also please check that the other text I wrote is actually correct.

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[jonhnet]

@aconway: thanks for all the clarifications. All of these comments make and pass the tests. :v) Please take another peek.

[jonhnet]

@ajhconway gentle ping on this collection of comments. Thanks!