R1:91447c54bce0

Write 500 words on how to restart webservers

Summary:
Fixes T9874.

- Stop using the phrase "restart your webserver". Instead, say "restart Phabricator".
- Write a document explaining that "Restart Phabricator" means to restart all of the server processes, depending on how your configuration is set up, and approximately how to do that.
- Link to this document.
- In places where we are not specifically giving instructions and the user isn't expected to do anything, be intentionally vague so as to…