Sysadmin doc should mention we fork to lock under Unix

[james]

Sysadmins who pay attention to a Xapian install often are confused by either the forking behaviour or the presence of a child process during locking. Both should be explicitly mentioned in the documentation for sysadmins, to explain why it is a good thing.

We should perhaps also mention problems that could prevent the locking from working, such as resource limits and capability restrictions.