Improve recent doc fix for cl-reduce

* lisp/emacs-lisp/cl-seq.el: (cl-reduce): Clarify treatment of
:INITIAL-VALUE when :FROM-END is non-nil.  Improve wording when SEQ
is empty and :INITIAL-VALUE is unspecified (bug#37400).  Describe
behavior when SEQ comprises a single element.

lisp/emacs-lisp/cl-seq.el

@@ -129,14 +129,16 @@ second element of SEQ, then calling FUNCTION with that result and
 the third element of SEQ, then with that result and the fourth
 element of SEQ, etc.
 
-If :INITIAL-VALUE is specified, it is added to the front of SEQ.
-If SEQ is empty, return :INITIAL-VALUE and FUNCTION is not
-called.
-
-If SEQ is empty and no :INITIAL-VALUE is given, then the function
-is called with zero arguments, and reduce returns whatever
-function does. This is the only case where the function is called
-with other than two arguments.
+If :INITIAL-VALUE is specified, it is logically added to the
+front of SEQ (or the back if :FROM-END is non-nil).  If SEQ is
+empty, return :INITIAL-VALUE and FUNCTION is not called.
+
+If SEQ is empty and no :INITIAL-VALUE is specified, then return
+the result of calling FUNCTION with zero arguments.  This is the
+only case where FUNCTION is called with fewer than two arguments.
+
+If SEQ contains exactly one element and no :INITIAL-VALUE is
+specified, then return that element and FUNCTION is not called.
 
 \n(fn FUNCTION SEQ [KEYWORD VALUE]...)"
   (cl--parsing-keywords (:from-end (:start 0) :end :initial-value :key) ()