Improve Office 365 configuration documentation

Closes #154

Author: simonrob

emailproxy.config

@@ -98,9 +98,12 @@ documentation = Accounts are specified using your email address as the section h
 	in the example below) in order to allow the proxy to refresh its access token on your behalf. The proxy will still
 	work if this parameter is not included, but you will need to re-authenticate extremely often (about once per hour).
 
-	- If your Office 365 configuration requires a tenant ID, place it in both `permission_url` and `token_url` in place
-	of `common` in the example below. For more detail about this, and guides for setting up your desktop app API client,
-	see the documentation at https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app.
+	- By default, new Azure AD clients are accessible only within your own tenant. If you are registering a new client
+	to use with the proxy (and do not want to make it available outside your own organisation) you will need to replace
+	`common` with your tenant ID in the Office 365 `permission_url` and `token_url` values below. Alternatively, you can
+	reuse credentials from an existing client registration (see the proxy's README.md file), or configure your client
+	as a multi-tenant application. For more detail about this, and guides for setting up your desktop app client, see
+	the documentation at https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app.
 
 	- Office 365 shared mailboxes are supported: add an account entry here using the email address of the shared
 	mailbox as the account name. When asked to authenticate, log in as the user that access has been delegated to.