Update wording of help text

This makes some general changes to the wording of the help text for style. In particular:

* Uses "Homebrew" and "Tigerbrew" over "HomeBrew" and "TigerBrew"
* Normalizes capitalization
* Removes what looks like a duplicate `brew doctor` block, and softens the wording (since `doctor` warnings aren't errors)
* Recommends the official uninstaller script