[3.12] gh-64588: Clarify the difference between mu and xbar in statistics docs (GH-117333) (#118080)

gh-64588: Clarify the difference between mu and xbar in statistics docs (GH-117333)

Thanks Davin Potts for the clarification idea.
(cherry picked from commit fefd5d97111364afa027ae580c3244f427dda59d)

Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>