diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1f54372..f5ea078 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -7561,7 +7561,11 @@
 		If {input} is given and is a |List| it is written to the file
 		in a way |writefile()| does with {binary} set to "b" (i.e.
 		with a newline between each list item with newlines inside
-		list items converted to NULs).  
+		list items converted to NULs).
+		When {input} is given and is a number that is a valid id for
+		an existing buffer then the content of the buffer is written
+		to the file line by line, each line terminated by a NL and
+		NULs characters where the text has a NL.
 
 		Pipes are not used, the 'shelltemp' option is not used.