At it's simplest, presuming you have access to this HTML file, just edit it with a text editor. Simply insert your comments between <i>Your comments here</i>
As you mention a user friendly admin page I presume you want to do this editing across the web. Again at its simplest you want a script to read the file you want to edit and then output a web page containing a text area. This text area is set to the contents of the file to be edited so you get an editable copy of the file in your browser. You then insert your comments wherever you want. When you press submit this text area is sent back to the script where it is used to rewrite the original file. Thus you have edited the file over the web. This is the basic HTML your script would need to output.
<html>
<head>
<title>Admin Page</title>
</head>
<body onLoad="document.form.text.focus()">
<form method="POST" name="form" action="/cgi-bin/myscript.pl">
<p>
<textarea rows="20" name="text" cols="75">
File text to edit goes here via variable
</textarea>
</p>
<p>
<input type="submit" value="Submit" name="submit">
<input type="reset" value="Reset" name="reset">
</p>
</form>
</body>
</html>
It is hard to be much more specific given your somewhat vague request. I'm sure you will get more feedback if you can be more specific as to the details.
cheers
tachyon | 