diff --git a/runtime/doc/channel.txt b/runtime/doc/channel.txt
index 01a9e36..6705c59 100644
--- a/runtime/doc/channel.txt
+++ b/runtime/doc/channel.txt
@@ -628,15 +628,18 @@
 		Start logging channel activity to {fname}.
 		When {fname} is an empty string: stop logging.
 
-		When {mode} is omitted or "a" append to the file.
-		When {mode} is "w" start with an empty file.
+		When {mode} is omitted or contains "a" or is "o" then append
+		to the file.
+		When {mode} contains "w" and not "a" start with an empty file.
+		When {mode} contains "o" then log all terminal output.
+		Otherwise only some interesting terminal output is logged.
 
 		Use |ch_log()| to write log messages.  The file is flushed
 		after every message, on Unix you can use "tail -f" to see what
 		is going on in real time.
 
 		To enable the log very early, to see what is received from a
-		terminal during startup, use |--log|: >
+		terminal during startup, use |--log| (this uses mode "ao"): >
 			vim --log logfile
 <
 		This function is not available in the |sandbox|.
diff --git a/runtime/doc/starting.txt b/runtime/doc/starting.txt
index f5b33a8..ec095a4 100644
--- a/runtime/doc/starting.txt
+++ b/runtime/doc/starting.txt
@@ -346,7 +346,7 @@
 <
 --log {filename}					*--log*
 		Start logging and write entries to {filename}.
-		This works like calling `ch_logfile({filename}, 'a')` very
+		This works like calling `ch_logfile({filename}, 'ao')` very
 		early during startup.
 		{only available with the +channel feature}