(Minibuffer Contents): Add minibuffer-completion-contents.

lispref/minibuf.texi

@@ -1836,6 +1836,15 @@ This is like @code{minibuffer-contents}, except that it does not copy text
 properties, just the characters themselves.  @xref{Text Properties}.
 @end defun
 
+@defun minibuffer-completion-contents
+@tindex minibuffer-completion-contents
+This is like @code{minibuffer-contents}, except that it when point is
+not at the end of the minibuffer, it doesn't return the entire
+contents of the minibuffer.  It returns the contents of the minibuffer
+just before point.  That is what completion commands operate on.
+@xref{Minibuffer Completion}.
+@end defun
+
 @defun delete-minibuffer-contents
 @tindex delete-minibuffer-contents
 This function erases the editable contents of the minibuffer (that is,