Look in your language files. It's part of the array $LANG03.
Text Formatted Code
$LANG03 = array(
// snip
18 => 'Important Stuff',
19 => 'Please try to keep posts on topic.',
20 => 'Try to reply to other people comments instead of starting new threads.',
21 => 'Read other people\'s messages before posting your own to avoid simply duplicating what has already been said.',
22 => 'Use a clear subject that describes what your message is about.',
23 => 'Your email address will NOT be made public.',
You could also do something about it in public_html/layout/professional/comment/commentform.thtml and commentform_advanced.thtml:
Text Formatted Code
<td colspan="2"><b>{lang_importantstuff}</b><ul>
<li>{lang_instr_line1}</li>
<li>{lang_instr_line2}</li>
<li>{lang_instr_line3}</li>
<li>{lang_instr_line4}</li>
<li>{lang_instr_line5}</li></ul>