Describe the lock behaviour for an expiration of 0

Related to JCBC-2146 I note that the documentation isn't clear about the case when a lock expiry time is 0.

For example in the following page (this is not an exhaustive list of affected pages) we mention what happens if the client issues a lock greater than the max (> 30s), but not what happens if 0.

https://docs.couchbase.com/php-sdk/current/howtos/concurrent-document-mutations.html#pessimistic-locking

If a client does issue a 0 expiry time, the server will use the default (which is 15s).