Make documentation @api private by default.

[reitermarkus]

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Our API is mostly private, so make anything that is unmarked @api private by default.

I added a brew rubydoc --open command to view docs locally.

@Homebrew/maintainers, please check if you find anything that is currently unmarked but used outside of this repo, and add the following accordingly:

• @api public: Usable in formulae/casks, or by third-party taps.
• @api internal: Usable by official taps.
• @api private: Usable in Homebrew/brew.
• @private: Only used for things that can't be made private.

[MikeMcQuaid]

Love this idea!

• https://github.com/Homebrew/homebrew-bundle/tree/master/spec/stub is pretty comprehensive for homebrew-bundle as it has 100% line coverage
• https://github.com/Homebrew/homebrew-test-bot/tree/master/spec/stub is less so but should provide a decent starting point and you can probably click around for the rest
• https://github.com/Homebrew/homebrew-services/tree/master/spec/stub is much less so but, again, can probably just click around the tap

@api public: Usable in formulae/casks, or by third-party taps.
@api internal: Usable by official taps.
@api private: Usable in Homebrew/brew.
@Private: Only used for things that can't be made private.

Would be great to explicitly document this somewhere.

I realise this is probably actively impossible but: would be great to figure out if this is enforceable at all through checks, too.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[MikeMcQuaid]

@reitermarkus Would love you to rebase this and we can get it merged when you get a bit!

[MikeMcQuaid]

Good to merge when you are 👍🏻

[carlocab]

This class is used in Homebrew/test-bot, I believe.

[reitermarkus]

Couln't find it in test-bot.

[carlocab]

https://github.com/Homebrew/homebrew-test-bot/blob/390f6b1ee21f4e017b6fff1d9e1d8b3550262468/lib/tests/formulae_dependents.rb#L117

[MikeMcQuaid]

Looks good! I'm sure we'll need to adjust some of this over time but let's get it merged ASAP to avoid conflicts. Great work @reitermarkus!