Please clarify `timeout` in the docs

[mscheper]

The README docs don't explain exactly what the timeout is, nor details about how it works.

I gather that, if the decorated function is called with the same arguments twice within timeout seconds, it will retrieve the return value from the cache, instead of invoking the function. But that does leave these questions:

• Does the second call to the function reset the timeout? i.e. If the function is called a third time with the same arguments, more than timeout seconds after the first call but less than timeout seconds after the second, are the values retrieved from the cache, or is the function invoked?
• Is the cached value automatically cleared after timeout seconds, or only after a subsequent call to the function, with the same arguments?