Add information about disabling multi-tenant (#2805)

Also clean up the language a bit.

Signed-off-by: Austin McKinley <[email protected]>

Author: amckinley

docs/production/auth.md

@@ -10,11 +10,15 @@ on each request. They trust this value completely: if you need to
 protect your Cortex installation from accidental or malicious calls
 then you must add an additional layer of protection.
 
-Typically this means you run Cortex behind a reverse proxy, and ensure
-that all callers, both machines sending data over the remote_write
+Typically this means you run Cortex behind a reverse proxy, and you must
+ensure that all callers, both machines sending data over the `remote_write`
 interface and humans sending queries from GUIs, supply credentials
 which identify them and confirm they are authorised.
 
-When configuring the remote_write API in Prometheus there is no way to
+When configuring the `remote_write` API in Prometheus there is no way to
 add extra headers. The user and password fields of http Basic auth, or
-Bearer token, can be used to convey tenant ID and/or credentials.
+Bearer token, can be used to convey the tenant ID and/or credentials.
+
+To disable the multi-tenant functionality, you can pass the argument
+`-auth.enabled=false` to every Cortex component, which will set the OrgID
+to the string `fake` for every request.