patch 8.2.1545: ch_logfile() is unclear about closing when forking

Problem:    ch_logfile() is unclear about closing when forking.
Solution:   Adjust the log messages.
diff --git a/src/os_unix.c b/src/os_unix.c
index 73caa1a..b58138b 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -4683,8 +4683,10 @@
 
 # ifdef FEAT_JOB_CHANNEL
 	    if (ch_log_active())
-		// close the log file in the child
+	    {
+		ch_log(NULL, "closing channel log in the child process");
 		ch_logfile((char_u *)"", (char_u *)"");
+	    }
 # endif
 
 	    if (!show_shell_mess || (options & SHELL_EXPAND))