Acceptable cases for `:target`

@caskroom/maintainers Stemming from the conversation in #6375, this is the proposal (with appropriate changes) for the rules pertaining to the use of `:target`. Its use with `app` has been tightened, while its use with `binary` has remained liberal in comparison (if it should remain so is also something that should be agreed upon).

Regarding apps, I’ve tried to keep the message consistent between them, and found three main reasons for renaming with `:target`:

+ For clarity: app name is inconsistent with its branding.
+ For consistency: app name is different in the Finder and in a shell.
+ To avoid conflict.