Updated Sendfile documentation for OwnCloud 7

* Removed mentions of temporary zip filepath - This is no longer relevant because of [ZipStreamer](owncloud/core@b35cd54)
Updated to reflect NGINX Sendfile changes 

Updated to reflect NGINX Sendfile changes owncloud/core#7838
Updated Zip generation wording
Added comments in the config.
Merge pull request #306 from josh4trunks/master

Updated X-Sendfile Documentation