Add documentation for `provisioning_profile` attr of `ios_unit_test`

[kkpattern]

For an ios_unit_test to run on a real device, the provisioning_profile needs to be set. However, the documentation of ios_unit_test doesn't mention this. Some users may not know about this. https://github.com/bazelbuild/tulsi/issues/266 .

If it's ok, I'd like to create a PR. But I don't know if any other attrs need to be mentioned. For our project, the provisioning_profile is the only one required.

[keith]

Documenting this would be great, I don't think others are required

[kkpattern]

Cool. Does ios_ui_test also require provisioning_profile to run on a real device? I assume that's the case, but we don't have UI tests, so I can't verify it. If needed I can also add documentation for ios_ui_test.

[keith]

Yep I think they are needed for anything on device