The `label_flag`, `label_setting` rules have no reference documentation

Description of the bug:

The label_flag and label_setting built-in rules are discussed in https://bazel.build/extending/config#label-typed-build-settings, but have no reference documentation listing their attributes. It should probably be documented at https://bazel.build/reference/be/general. Only by searching around the internet for code examples did I find, for example, that the label_flag rule has a help attribute!

The situation is even more dire with label_setting, because there is no example code showing how one might use it.

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

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Which operating system are you running Bazel on?

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What is the output of bazel info release?

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If bazel info release returns development version or (@non-git), tell us how you built Bazel.

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What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

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Have you found anything relevant by searching the web?

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Any other information, logs, or outputs that you want to share?

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