patch 8.2.5056: the channel log only contains some of the raw terminal output

Problem:    The channel log only contains some of the raw terminal output.
Solution:   Add the "o" flag to log all terminal output.  Use it for "--log".

runtime/doc/channel.txt

@@ -628,15 +628,18 @@ ch_logfile({fname} [, {mode}])					*ch_logfile()*
 		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|.

runtime/doc/starting.txt

@@ -346,7 +346,7 @@ a slash.  Thus "-R" means recovery and "-/R" readonly.
 <
 --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}