Documentation for @bazel_tools//src/conditions

@laszlocsomor used this technique in a bzl file to test if we're on Windows:

```python
        is_windows = select({
            "@bazel_tools//src/conditions:host_windows": True,
            "//conditions:default": False,
        }),
```

I didn't find anything about `@bazel_tools//src/conditions` in the documentation.