Add sections on locking and remote shares to unix/windows

This comes after several reports on the mailing lists of NFS and CIFS installs
failing.

 # Questions #

 * Is the section obvious enough? Do we need a red blinking banner?
 * Are all of the terms well-enough understood? Do we need links?
 * Are there any tests that can be performed to check locking?
 * Should we also add a section on latency?

/cc @chris-allan