snap disconnect <snapname>: prints a confusing error

[Paweł Stołowski]

The message for "snap disconnect foosnap:" commands prints a confusing message suggesting that "snap disconnect foosnap" syntax is supported (but it's not as shown below):

$ sudo snap disconnect skype:
error: invalid value: "skype:" (want snap:name or snap)

$ sudo snap disconnect skype
error: please provide the plug or slot name to disconnect from snap "skype"

For the record, according to help the following syntax is supported:

$ snap disconnect --help
Usage:
  snap disconnect [disconnect-OPTIONS] [<snap>:<plug>] [<snap>:<slot>]

The disconnect command disconnects a plug from a slot.
It may be called in the following ways:

$ snap disconnect <snap>:<plug> <snap>:<slot>

Disconnects the specific plug from the specific slot.

$ snap disconnect <snap>:<slot or plug>

Disconnects everything from the provided plug or slot.
The snap name may be omitted for the core snap.

(...)

[Paweł Stołowski (stolowski)]

Related PR: https://github.com/snapcore/snapd/pull/10970