hg: lambda/lambda/jdk: Docs.

[Brian Goetz]

> Or not being there at all. Couldn't OfDouble/OfInt/OfLong directly
> extend Iterator and also be placed inside it? Are you anticipating to
> add any default methods to PrimitiveIterator directly?

The motivation was not that we might add methods, but that other code in 
the implementation might do:

<U, T extends PrimitiveIterator<U>> foo(U iterator).

In fact, earlier versions of the code did exactly that.