Documentation updates

[Rylan12]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

---

This PR makes two documentation changes:

1. Cleanup the text around the cask options in the manpage/documentation. brew cask is now disabled so these options don't apply to its subcommands. Additionally, the cask options are only used in the install, reinstall, and upgrade commands rather than all commands that accept --cask.
2. Clarifies the Maintainer Guidelines by adding a chart explaining when to use squash & merge versus rebase & merge.

[BrewTestBot]

Review period will end on 2020-12-30 at 18:05:01 UTC.

[BrewTestBot]

Review period will end on 2020-12-30 at 18:05:01 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks @Rylan12!

[Rylan12]

Thanks @samford and @MikeMcQuaid for the reviews!