[monitoring-plugins] Add a comment to opttest to explain the purpose

[Lorenz Kästle]

 Module: monitoring-plugins
 Branch: master
 Commit: 0d0f4ee7e30dcd2c5f9f0c53400ed6b2a9bc14a2
 Author: Lorenz Kästle <12514511+RincewindsHat at users.noreply.github.com>
   Date: Wed Feb 19 19:46:37 2025 +0100
    URL: https://www.monitoring-plugins.org/repositories/monitoring-plugins/commit/?id=0d0f4ee7

Add a comment to opttest to explain the purpose

---

 tools/opttest.pl | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/opttest.pl b/tools/opttest.pl
index 56a5c88f..98213082 100755
--- a/tools/opttest.pl
+++ b/tools/opttest.pl
@@ -3,6 +3,10 @@ use strict;
 use warnings;
 use Test;
 
+# This script (when executed from the monitoring plugins top level directory)
+# executes all the plugins with -h, --help, -V and --version to verify that
+# all of them exit properly with the state UNKNOWN (3)
+
 use vars qw($dir $file $prog $idx $state $output %progs @dirs);
 
 my $tests = 0;