patch 8.2.4122: ":command Cmd" does not show custom completion argument

Problem:    ":command Cmd" does not show custom completion argument.
Solution:   Show the completion argument when using ":verbose".
diff --git a/src/usercmd.c b/src/usercmd.c
index 70f45aa..808d36c 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -548,6 +548,13 @@
 		{
 		    STRCPY(IObuff + len, command_complete[j].name);
 		    len += (int)STRLEN(IObuff + len);
+		    if (p_verbose > 0 && cmd->uc_compl_arg != NULL
+					    && STRLEN(cmd->uc_compl_arg) < 200)
+		    {
+			IObuff[len] = ',';
+			STRCPY(IObuff + len + 1, cmd->uc_compl_arg);
+			len += (int)STRLEN(IObuff + len);
+		    }
 		    break;
 		}