Clarify configuration file token caching

Author: simonrob

emailproxy.config

@@ -80,10 +80,10 @@ documentation = Accounts are specified using your email address as the section h
 	OAuth 2.0 client registrations with the mail access scope, so reusing existing credentials is the only option here.
 
 	Once the proxy is correctly configured, after the first successful use of an account its access token details will
-	be automatically cached for future use, encrypted with the IMAP/POP/SMTP password you used in your email client.
-	By default this configuration file is reused for caching, but you can specify a different location or method using
-	the proxy's`--cache-store` parameter. You do not need to add or edit cached values manually (i.e., `token_salt`,
-	`access_token`, `access_token_expiry`, `refresh_token` and `last_activity`); the proxy handles this for you.
+	be cached for future use, encrypted with the IMAP/POP/SMTP password you used in your email client. By default this
+	configuration file is reused for caching (so it must be writable), but you can specify a different location or
+	method using the proxy's`--cache-store` parameter. You do not need to add or edit cached values manually (i.e.,
+	`token_salt`, `access_token`, `access_token_expiry`, `refresh_token` and `last_activity`); the proxy handles this.
 
 	The password used in your email client is not used for authentication with the actual email server (this is done via
 	OAuth 2.0 in a web browser), so it can be different to your real account password, which is helpful for debugging.