diff --git a/pack.c b/pack.c
index 3413ec6c56..93577a9280 100644
--- a/pack.c
+++ b/pack.c
@@ -242,7 +242,8 @@ str_associated(VALUE str)
  *   H            | String  | hex string (high nibble first)
  *   h            | String  | hex string (low nibble first)
  *   u            | String  | UU-encoded string
- *   M            | String  | quoted printable, MIME encoding (see RFC2045)
+ *   M            | String  | quoted printable, MIME encoding (see also RFC2045)
+ *                |         | (text mode but input must use LF and output LF)
  *   m            | String  | base64 encoded string (see RFC 2045, count is width)
  *                |         | (if count is 0, no line feed are added, see RFC 4648)
  *   P            | String  | pointer to a structure (fixed-length string)