string.c (str_uminus): update doc for deduplication

As of r57698, String#-@ can return pre-existing strings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57813 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -2520,8 +2520,8 @@ str_uplus(VALUE str)
  *
  * If the string is frozen, then return the string itself.
  *
- * If the string is not frozen, then duplicate the string
- * freeze it and return it.
+ * If the string is not frozen, return a frozen, possibly pre-existing
+ * copy of it.
  */
 static VALUE
 str_uminus(VALUE str)