Incorrect docs for negative_caching_enabled

Today we had an issue with an origin which threw some 500 errors. We noticed that trafficserver cached the responses even though we had negative caching disabled. When we went to dig into this we noticed that the docs are a bit confusing (missing newline after NOTE and response code list) and somewhat incomplete-- if a response has cache control directives it will be cached regardless of response code.

This is my first commit to the docs, so hopefully I've done it correctly if not please let me know!