diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 5b383fd..5cb0bd9 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -6866,8 +6866,10 @@
 			readblob('/dev/ttyS0', 0, 10)
 <		When the file can't be opened an error message is given and
 		the result is an empty |Blob|.
-		When trying to read bytes beyond the end of the file the
-		result is an empty blob.
+		When the offset is beyond the end of the file the result is an
+		empty blob.
+		When trying to read more bytes than are available the result
+		is truncated.
 		Also see |readfile()| and |writefile()|.