Clarify in the docs that the cache is used throughout the whole execution of a fetch

Spotted in the gitter channel

The big thing for me was that it wasn't clear that the cache could be reused. It talks about the state monad and the cache being immutable which caused me to make some bad assumptions, even after diving into the code.

This should be more explicit and clear to not confuse people.

[tomjadams]

I'd also add that it can be reused between fetches also, i.e. if you have a cache (say ScalaCache) that you wrap & pass in to the fetch run, that cache can be used.