diff --git a/doc/contrib/coding_guide.rst b/doc/contrib/coding_guide.rst
index 57bf07de4d74..f939a17b2a94 100644
--- a/doc/contrib/coding_guide.rst
+++ b/doc/contrib/coding_guide.rst
@@ -16,7 +16,7 @@ C++ Coding Guideline
   * Each line of text may contain up to 100 characters.
   * The use of C++ exceptions is allowed.
 
-- Use C++14 features such as smart pointers, braced initializers, lambda functions, and ``std::thread``.
+- Use C++17 features such as smart pointers, braced initializers, lambda functions, and ``std::thread``.
 - Use Doxygen to document all the interface code.
 - We have some comments around symbols imported by headers, some of those are hinted by `include-what-you-use <https://include-what-you-use.org>`_. It's not required.
 - We use clang-tidy and clang-format. You can check their configuration in the root directory of the XGBoost source tree.
diff --git a/python-package/xgboost/testing/data.py b/python-package/xgboost/testing/data.py
index 1aca269f60b3..369833623bef 100644
--- a/python-package/xgboost/testing/data.py
+++ b/python-package/xgboost/testing/data.py
@@ -388,6 +388,7 @@ def __init__(self, eta: float) -> None:
         exam_prob = np.array(
             [0.68, 0.61, 0.48, 0.34, 0.28, 0.20, 0.11, 0.10, 0.08, 0.06]
         )
+        # Observation probability, encoding positional bias for each position
         self.exam_prob = np.power(exam_prob, eta)
 
     def sample_clicks_for_query(