gh-154942: Document the kde() data cache logic. (gh-157725)

(cherry picked from commit 5afcd82)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>

Author: rhettinger

Doc/library/statistics.rst

@@ -309,6 +309,11 @@ However, for reading convenience, most of the examples show sorted sequences.
    .. image:: kde_example.png
       :alt: Scatter plot of the estimated probability density function.
 
+   Because the returned ``f_hat`` function is typically called many times,
+   it caches the *data* for performance. To support dynamic datasets, this
+   cache automatically refreshes whenever the length of the *data* changes.
+   This allows new samples to be added as they become available.
+
    .. versionadded:: 3.13