[libc 1.0] Put a big warning to Linux's dirent docs

[safinaskar]

This fixes my bug #2669 .

We cannot change type for d_name field, because this will break tests. So the only thing we can do is to document dirent behavior.

The question remains what are exact rules for dealing with dirent, but this is left to Rust specification

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[safinaskar]

CI is failing, but it seems this is not because of my patch

[JohnTitor]

We should resolve the issue instead of emitting warnings on 1.0 as we can introduce breaking changes there.