-The meaning of COMMON-SUBSTRING is the same as in `display-completion-list'.
-Typing SPC flushes the completions buffer."
+Typing SPC flushes the completions buffer.
+
+The optional argument COMMON-SUBSTRING, if non-nil, should be a string
+specifying a common substring for adding the faces
+`completions-first-difference' and `completions-common-part' to
+the completions."