Added note about refresh and POSIX behavior

[rickmcgeer]

Per the discussion in #647, I've added comments in the initialization block about the fact that refresh = False, the default, will not mimic POSIX behavior of the ls utility.

[martindurant]

I wouldn't presuppose here what the user's expectations are. Let me have a go, see what you think.

Suggested change

	*Note that caching creates divergences from the behavior of POSIX utilities such as 'ls'*. In a POSIX
	filesystem, ls always returns the current state of a directory; however, if `refresh = False` (the
	default), ls returns the state of the cache. `refresh = False` is the default because (unlike in a POSIX filesystem)
	listing directory contents of a GCS bucket can be expensive. Developers are advised to set `refresh = True`
	if exact POSIX behavior is desired.
	Note that directory listings are cached by default, because fetching those listings can be expensive. This is
	contrary to local filesystem behaviour. The cache will be cleared if writing from this instance, but it can
	become stale and return incorrect results if the storage is written to from another process/machine.
	If you anticipate this possibility, you can set the use_listings_cache and listings_expiry_time arguments
	to configure the caching, call `.invalidate_cache()` when required, or pass `refresh=True` to the
	various listing methods.