Make the docs for string.capitalize() match those of str.capitalize()

(which makes it more clear).
Closes SF bug #571767.

Doc/lib/libstring.tex

@@ -135,7 +135,7 @@ The functions defined in this module are:
 \end{funcdesc}
 
 \begin{funcdesc}{capitalize}{word}
-  Capitalize the first character of the argument.
+  Return a copy of \var{word} with only its first character capitalized.
 \end{funcdesc}
 
 \begin{funcdesc}{capwords}{s}