Installing an already-installed snap docs could be more helpful

[Stuart Langridge]

If I "snap install thingy --edge" when I already have thingy installed, it tells me to read the help for snap refresh.

snap "thingy" is already installed, see 'snap help refresh'

I can understand that maybe it shouldn't just go "ok!" and overwrite my existing app without checking, but it would be rather nicer if it said something a but more helpful, such as:

snap "thingy" is already installed in the "stable" channel. To change to "edge", "snap refresh thingy --edge", or see "snap help refresh" for more details.

(With consistent punctuation.)