Implement string-upcase and document it.

doc/utils.md

@@ -448,6 +448,13 @@ characters of the string.
 Returns a new string with all non-ASCII characters encoded as
 quoted-printable sequences.
 
+    (string-upcase str)
+
+* ```str``` - arbitrary string
+
+Returns the ```str``` with all characters converted to upper case
+using ```char-upcase```. Does not work with UTF-8.
+
 ### Tag
 
     (import util-tag)