Document the need to restart Phabricator after performing a restore

Summary:
Depending on how you perform a restore, APC (or, e.g., running daemon processes) might be poisoned with out-of-date caches.

Add a note to advise installs to restart after restoring data.

See also lengthy fishing expedition support thread.

Test Plan: Read the text.

Reviewers: chad, amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18180