Document exception chaining (#3855)

This is already documented as a user note, but the note does not explain
what happens if both exceptions already have a previous exception.

Here is what I believe is the related piece of code:

```
/* Chain potential exception from wrapping finally block */
if (Z_OBJ_P(fast_call)) {
	if (ex) {
		if (zend_is_unwind_exit(ex) || zend_is_graceful_exit(ex)) {
			/* discard the previously thrown exception */
			OBJ_RELEASE(Z_OBJ_P(fast_call));
		} else {
			zend_exception_set_previous(ex, Z_OBJ_P(fast_call));
		}
	} else {
		ex = EG(exception) = Z_OBJ_P(fast_call);
	}
}
```

The note: https://www.php.net/manual/en/language.exceptions.php#129177

Author: greg0ire

language/exceptions.xml

@@ -79,6 +79,12 @@
     is executed. Additionally, if the &finally; block also contains a &return; statement,
     the value from the &finally; block is returned.
    </para>
+   <para>
+    Another notable interaction is between an exception thrown from within a &try; block,
+    and an exception thrown from within a &finally; block. If both blocks throw an exception,
+    then the exception thrown from the &finally; block will be the one that is propagated,
+    and the exception thrown from the &try; block will used as its previous exception.
+   </para>
   </sect1>
 
  <sect1 annotations="chunk:false" xml:id="language.exceptions.exception-handler">
@@ -359,6 +365,38 @@ try {
     <screen>
 <![CDATA[
 It did not work
+]]>
+    </screen>
+   </example>
+   <example>
+    <title>Exception in try and in finally</title>
+    <programlisting role="php">
+<![CDATA[
+<?php
+
+try {
+    try {
+        throw new Exception(message: 'Third', previous: new Exception('Fourth'));
+    } finally {
+        throw new Exception(message: 'First', previous: new Exception('Second'));
+    }
+} catch (Exception $e) {
+    var_dump(
+        $e->getMessage(),
+        $e->getPrevious()->getMessage(),
+        $e->getPrevious()->getPrevious()->getMessage(),
+        $e->getPrevious()->getPrevious()->getPrevious()->getMessage(),
+    );
+}
+]]>
+    </programlisting>
+    &example.outputs;
+    <screen>
+<![CDATA[
+string(5) "First"
+string(6) "Second"
+string(5) "Third"
+string(6) "Fourth"
 ]]>
     </screen>
    </example>