diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index eab508af3a..cc45a71f57 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -477,6 +477,10 @@ project-read-file-name-function
 (defun project--read-file-cpd-relative (prompt
                                         all-files &optional predicate
                                         hist default)
+  "Read a file name, prompting with PROMPT.
+ALL-FILES is a list of possible file name completions.
+PREDICATE, HIST, and DEFAULT have the same meaning as in
+`completing-read'."
   (let* ((common-parent-directory
           (let ((common-prefix (try-completion "" all-files)))
             (if (> (length common-prefix) 0)