the two strings is the actual completion, the second string serves as
annotation.
-@var{common-substring} represents a string common prefix between all
-completions. @var{common-substring} is used to put faces on
-@samp{*Completions*} to enhance visual feedback of the buffer;
-@code{display-completion-list} use faces to highlight what all completions
-have in common and where they begin to differ. @var{common-substring}
-specifies the common part.
+The argument @var{common-substring} is the prefix that is common to
+all the completions. With normal Emacs completion, it is usually the
+same as the string that was completed. @code{display-completion-list}
+uses this to highlight text in the completion list for better visual
+feedback. This is not needed in the minibuffer; for minibuffer
+completion, you can pass @code{nil}.
This function is called by @code{minibuffer-completion-help}. The
most common way to use it is together with