"snap interface --help" is misleading WRT "--attrs" option

[Robert P. J. Day]

"snap interface --help" explains the "--attrs" option this way:

  [interface command options]
      --attrs Show interface attributes

But it doesn't show the attributes of that interface; rather, it lists the attributes and values of the actual plugs and slots of that interface on the system. Perhaps the explanation might read:

"Show interface attributes and values for existing plugs and slots"

or something equivalent.