diff --git a/src/abbrev.c b/src/abbrev.c
index 1cf06946631..41e329ac95d 100644
--- a/src/abbrev.c
+++ b/src/abbrev.c
@@ -540,7 +540,8 @@ is inserted.  Otherwise the description is an expression,
 a call to `define-abbrev-table', which would
 define the abbrev table NAME exactly as it is currently defined.
 
-Abbrevs marked as "system abbrevs" are omitted.  */)
+Abbrevs marked as "system abbrevs" are normally omitted.  However, if
+READABLE is non-nil, they are listed.  */)
      (name, readable)
      Lisp_Object name, readable;
 {