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 avoid being misleading.

Test Plan:

• Read document.
• Hit "exetnsion" and "PHP config" setup checks, got "restart Phabricator" with documentation links in both cases.

Reviewers: chad

Maniphest Tasks: T9874

Differential Revision: https://secure.phabricator.com/D14636