Make it clear that manually creating an encrypted volume on the Tails device is not supported

Tails/Wheezy protects the boot device against write access via udisks by the desktop user (#6172 (closed)), so if one follows doc/encryption_and_privacy/encrypted_volumes and tries to manually create an encrypted volume on the boot device, they’ll be asked an administration password.

I see two ways out of it:

1. Add a note, on top of that page, stating that manually creating an encrypted volume on the Tails device is not supported, and pointing to the doc about persistence.
2. Mention, on top of that page, that one needs to set an administration password first.

I prefer the first one, but feel free to retitle this ticket if the people responsible for the doc prefer the latter.

Feature Branch: doc/wheezy

Parent Task: #6980 (closed)

Original created by @intrigeri on 7044 (Redmine)