need to document the supported syntax for dots, subdots and branch pinning

[lukemarsden]

You can specify subdots with dot.subdot. You can pin branches with dot@branch. How do you pin the branch of a subdot? What does branch pinning even mean? How does this interact with Kubernetes?

[alaric-dotmesh]

In kubernetes, you get separate PVC annotations for dot name, dot namespace, subdot name - and if we fix dotmesh#176, also branch name.

What we need to make clear is that we have a syntax for volume names as distinct from dot names. IIRC this is currently only used when doing docker -v, but it may crop up in dm arguments and in how we display names in the hub UI as well.

We need to document that syntax (where? It kind of fits in lots of places... Does it need a page of its own?) and then refer to that document wherever appropriate.

(Also, IMHO, we should make it accept both dot.subdot@branch and dot@branch.subdot to avoid annoying people, but output dot@branch.subdot as the canonical form as it's mnemonic of the logical structure)

Also note that branch pinning @ syntax might, in future, also be used to access commits as read-only volumes; we should leave that door open, unless we want to use a separate character for that.