doc UPDATE clarify invalid characters
diff --git a/src/common.c b/src/common.c
index 38f51ea..f3b1d6b 100644
--- a/src/common.c
+++ b/src/common.c
@@ -258,6 +258,7 @@
                 (value != 0x09) &&
                 (value != 0x0a) &&
                 (value != 0x0d)) {
+            /* valid UTF8 but not YANG string character */
             return LY_EINVAL;
         }