Update doc's explaining that 404 is by design

[deweydb]

I got stuck for a while wondering why i was getting 404s when testing the cache purge via CURL. Then i realized that if there is no cached doc for that url, then you get a 404. It should mention this somewhere in the docs so people don't get confused.

[larssn]

Thanks. Was wondering what was going on with all those 404s.