Document new utf-8 functions.

doc/utils.md

@@ -428,6 +428,26 @@ Returns a pair of strings where the ```car``` of the pair is the first
 token in the ```str``` given and ```cdr``` is a string with the
 remainder with leading whitespace removed.
 
+    (string-utf8? str)
+
+* ```str``` - arbitrary string
+
+Returns ```#t``` if given string ```str``` contains UTF-8 characters.
+
+    (string->list/utf8 str)
+
+* ```str``` - arbitrary string
+
+Returns a list of strings representing individual (possibly UTF-8)
+characters of the string.
+
+    (string->qp str)
+
+* ```str``` - arbitrary string
+
+Returns a new string with all non-ASCII characters encoded as
+quoted-printable sequences.
+
 ### Tag
 
     (import util-tag)