Add DStr docs.

Small test changes

test/test_dstr.c

@@ -65,9 +65,8 @@ START_TEST (test_dstr_own_steal)
 END_TEST
 
 START_TEST (test_dstr_trim)
-    struct DStr str;
+    struct DStr str = UC_DSTR_STATIC_INIT;
 
-    uc_dstr_init(&str);
     uc_dstr_put_str(&str, " \t  \nHello !\r\n");
     uc_dstr_trim(&str);
     ck_assert_str_eq(uc_dstr_str(&str), "Hello !");