--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2041,7 +2041,7 @@ winrestcmd()			String	returns command to
 winrestview( {dict})		none	restore view of current window
 winsaveview()			Dict	save view of current window
 winwidth( {nr})			Number	width of window {nr}
-writefile( {list}, {fname} [, {binary}])
+writefile( {list}, {fname} [, {flags}])
 				Number	write list of lines to file {fname}
 xor( {expr}, {expr})		Number  bitwise XOR
 
@@ -6555,14 +6555,20 @@ winwidth({nr})						*winwidth()*
   :endif
 <
 							*writefile()*
-writefile({list}, {fname} [, {binary}])
+writefile({list}, {fname} [, {flags}])
 		Write |List| {list} to file {fname}.  Each list item is
 		separated with a NL.  Each list item must be a String or
 		Number.
-		When {binary} is equal to "b" binary mode is used: There will
+		When {flags} contains "b" then binary mode is used: There will
 		not be a NL after the last list item.  An empty item at the
 		end does cause the last line in the file to end in a NL.
-		All NL characters are replaced with a NUL character.
+
+		When {flags} contains "a" then append mode is used, lines are
+		append to the file: >
+			:call writefile(["foo"], "event.log", "a")
+			:call writefile(["bar"], "event.log", "a")
+>
+<		All NL characters are replaced with a NUL character.
 		Inserting CR characters needs to be done before passing {list}
 		to writefile().
 		An existing file is overwritten, if possible.