docs: clarify Formula location in Tap repositories
docs: clarify Formula location in Tap repositories

Make clear that Formula locations cannot be mixed, they are mutually
exclusive. This avoids the (false) implication that top-level Formula
can override those in the Formula subdirectory.

This makes no change in recommendations following the original
discussion in https://github.com/Homebrew/legacy-homebrew/pull/41858

JCount/brew,Homebrew/brew,konqui/brew,claui/brew,konqui/brew,mahori/brew,MikeMcQuaid/brew,vitorgalvao/brew,sjackman/homebrew,reitermarkus/brew,JCount/brew,konqui/brew,JCount/brew,nandub/brew,claui/brew,EricFromCanada/brew,sjackman/homebrew,claui/brew,sjackman/homebrew,MikeMcQuaid/brew,nandub/brew,JCount/brew,EricFromCanada/brew,reitermarkus/brew,mahori/brew,reitermarkus/brew,EricFromCanada/brew,vitorgalvao/brew,MikeMcQuaid/brew,mahori/brew,nandub/brew,sjackman/homebrew,Homebrew/brew,claui/brew,Homebrew/brew,reitermarkus/brew,EricFromCanada/brew,MikeMcQuaid/brew,nandub/brew,vitorgalvao/brew,vitorgalvao/brew,mahori/brew,Homebrew/brew,konqui/brew