patch 8.2.4742: there is no way to start logging very early in startup

Problem:    There is no way to start logging very early in startup.
Solution:   Add the --log argument.  Include the date in the start message in
            the log file.  Avoid a duplicate message when forking.  Log an
            executed shell command.
diff --git a/runtime/doc/channel.txt b/runtime/doc/channel.txt
index a306abb..01756f1 100644
--- a/runtime/doc/channel.txt
+++ b/runtime/doc/channel.txt
@@ -633,8 +633,8 @@
 		is going on in real time.
 
 		To enable the log very early, to see what is received from a
-		terminal during startup, use |--cmd|: >
-			vim --cmd "call ch_logfile('logfile', 'w')"
+		terminal during startup, use |--log|: >
+			vim --log logfile
 <
 		This function is not available in the |sandbox|.
 		NOTE: the channel communication is stored in the file, be