Audit.log is not always available

We state that the audit log file always exists.
https://developer.couchbase.com/documentation/server/5.1/security/security-audit-targets.html

The specific sentence that needs changing is:
"A default file, named audit.log is always available; and logged events are written to this file."

The issue is that the file is does not always exist. A code change is in place to ensure that the file "almost always" exists.

See code change: http://review.couchbase.org/#/c/90599/

But even with the change it will not be possible to guarantee that it exists 100% of the time.
The reason is when we rotate the file we close the current one, rename and open the new
one.

Therefore sentence needs to change to something like.

"Logged events are written to a default file, named audit.log. The file is created when auditing is enabled and is available until the file rotates where upon its is closed and renamed and open the new one is created."