docs: Mention `layering_check` in header inclusion check docs

[fmeum]

The layering_check feature and toolchain support are required to realize header dependency checking in Bazel. If enabled, this also reports errors for explicitly included transitive, non-direct headers.

Fixes #17055 Fixes #15632

[fmeum]

@oquenchil Could you verify that what I wrote (and deleted) is actually correct? I have seen layering_check work on macOS, but that might have been with the non-Xcode toolchain. If the Xcode toolchain doesn't support layering_check, I would update the docs to reflect that (or try to add support).

[oquenchil]

Yes, this is correct, @hlopko added support for this a couple of years ago.

[fmeum]

@oquenchil @sgowroji Friendly ping, does this still need to be imported?

[sgowroji]

Its imported at https://github.com/bazelbuild/bazel/commit/90eee433f002c8b7a9f4fa6c4d65cb0479296876