search UI should be clear when forks or archived repositories were excluded

Created by: slimsag

A user at https://app.hubspot.com/contacts/2762526/company/407948923 complained that they were not seeing expected results. This was due to the fact that forks are no longer included by default as of 3.14.

I think it is reasonable that this might surprise some users and lead to a negative impression ("sometimes it doesn't show results that bitbucket's search does") so we should be clear about e.g. how many forks were excluded and the fact that you can add fork:yes to include them. The same should go for archived repositories.

cc @christinaforney for prioritization