How do you make a block comment in SVG

As SVG is XML, you can use XML style comments:

<!-- 
    comment 
-->

For example:

<g onclick = "setScale(1)">
    <rect id = "scale1" x = "120" y = "10" width = "30" height = "30"
        fill = "#ffc" stroke = "black"/>
    <!-- 
        this text describes middle rectangle
    -->
    <text x = "135" y = "30" text-anchor = "middle">M</text>
</g>

Or you can exclude some part of code:

<!--
     this group is disabled for testing    
<g onclick = "setScale(1)">
    <rect id = "scale1" x = "120" y = "10" width = "30" height = "30"
        fill = "#ffc" stroke = "black"/>
    <text x = "135" y = "30" text-anchor = "middle">M</text>
</g>
-->