Documenting the limits for restricted interfaces

[slimjim777]

There are some gotchas that need to be documented as the solution is not always obvious.

When using a snap with the restricted snapd-control interface, the snap cannot be installed from the store. The following error will be encountered:
ERROR installation not allowed by "snapd-control" plug rule of interface "snapd-control"

The restriction is set by https://github.com/snapcore/snapd/blob/master/interfaces/builtin/basedeclaration.go#L154, which means that the snap can only be installed from a local file.It is also possible to request the store reviewers to implement a snap-declaration that allows installation of this particular snap and the usage of snap-control.