Adjust subscription documentation for Silva Layout view

[Kit Blake]

Quoting from: https://bugs.edge.launchpad.net/dlcms/+bug/152635/comments/12, there are two points about documentation that need to be done. Please also make a link to the document in Manager that explains the use of MailDropHost.

OK, I used <silva root> + 'public/subscriptor' to access the
subscription form. I this case the ZMI based layout templates is used.
With <silva root> + '/subscriptor' the SilvaLayout based layout is used.
Thus, this problem is solved.

I suggest that the documentation on
http://www.infrae.com/products/silva/silva_docs_1_6/manager/subscription_config
should tell users to use <silva root> + '/subscriptor' if SilvaLayout
based layouts are used.

Apropos documentation, it would also be helpful if the Silva
documentation would contain a note that service_subscriptions_mailhost
needs to be configured to setup the subscription feature.