Add a note that the SSI set will no longer do HTML decoding between 2.2.x and 2.4.x.

Users that have an SSI page like the following would notice encoded entities in their response when migrating from 2.2.x to 2.4.x:

<!--#set var="message_entity" value="&lt;Hello &amp; goodbye!&gt;" -->
<p><!--#echo var="message_entity" --></p>

For example, the response they would see in 2.2.x would be:
<Hello & goodbye!>

while the response they would see in 2.4.x:
&lt;Hello &amp; goodbye!&gt;