Docs: non-cask binaries are not overwritten by --force

[vitorgalvao]

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This was decided in August but recently we had another issue because of it. Understandably, some people will expect --force to mean “trample everything”, so we should document when that isn’t the case.

[BrewTestBot]

Review period will end on 2021-01-08 at 21:26:39 UTC.

[BrewTestBot]

Review period skipped due to critical label.