Allow fallback when checking for guests only, and document it

[niladam]

Here's a summary of the changes in this commit:

• documents the LSCACHE_GUEST_ONLY config setting;
• allows for a default fallback in the middleware to false when checking for the guest only option (this is to prevent any issues when the user hasn't updated/added his config/env setting)'
• minor change to how the guest_only and Auth::check are determined as there's no need to use == true;

Improves https://github.com/litespeedtech/lscache-laravel/commit/243cbe985c93d1e011b7cff82bfd442e4afbba0c, which in turn addresses #7