Explicitely says only letters are supported as fast TODO selection keys.

* org.texi (Fast access to TODO states): Explicitely says only
letters are supported as fast TODO selection keys.

* org.el (org-todo-keywords): Ditto.

Thanks to Samuel Wales for pointing at this.

doc/org.texi

@@ -3924,9 +3924,10 @@ from @code{DONE} to @code{REPORT} in the example above.  See also
 @subsection Fast access to TODO states
 
 If you would like to quickly change an entry to an arbitrary TODO state
-instead of cycling through the states, you can set up keys for
-single-letter access to the states.  This is done by adding the section
-key after each keyword, in parentheses.  For example:
+instead of cycling through the states, you can set up keys for single-letter
+access to the states.  This is done by adding the selection
+letter@footnote{Only alphabetical letters are allowed for fast TODO selection
+keys.} after each keyword, in parentheses.  For example:
 
 @lisp
 (setq org-todo-keywords

lisp/org.el

@@ -2107,11 +2107,11 @@ cycling, see the manual.
 TODO keywords and interpretation can also be set on a per-file basis with
 the special #+SEQ_TODO and #+TYP_TODO lines.
 
-Each keyword can optionally specify a character for fast state selection
+Each keyword can optionally specify a letter for fast state selection
 \(in combination with the variable `org-use-fast-todo-selection')
-and specifiers for state change logging, using the same syntax
-that is used in the \"#+TODO:\" lines.  For example, \"WAIT(w)\" says
-that the WAIT state can be selected with the \"w\" key.  \"WAIT(w!)\"
+and specifiers for state change logging, using the same syntax that
+is used in the \"#+TODO:\" lines.  For example, \"WAIT(w)\" says that
+the WAIT state can be selected with the \"w\" letter.  \"WAIT(w!)\"
 indicates to record a time stamp each time this state is selected.
 
 Each keyword may also specify if a timestamp or a note should be
@@ -2125,7 +2125,7 @@ define X.  You may omit any of the fast-selection key or X or /Y,
 so WAIT(w@), WAIT(w/@) and WAIT(@/@) are all valid.
 
 For backward compatibility, this variable may also be just a list
-of keywords - in this case the interpretation (sequence or type) will be
+of keywords.  In this case the interpretation (sequence or type) will be
 taken from the (otherwise obsolete) variable `org-todo-interpretation'."
   :group 'org-todo
   :group 'org-keywords