Update chatbot documentation

Summary:
Remove's PhabricatorBotDifferentialNotificationHandler documentation and adds in:
PhabricatorBotFeedNotificationHandler
PhabricatorBotSymbolHandler
PhabricatorBotMacroHandler

Should have been included in D9477

Test Plan: Read it..?

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9479