Clarify incremental backup expiration.

The documentation was a bit misleading regarding how incremental backups are expired. Update the misleading part ("Differential and Incremental backups are count-based...") and move the explanation of how incremental expiration works out of differential expiration into the introductory paragraph.

Also add a note about how full backups are considered as differential for the purpose of expiration.

doc/xml/release/2025/2.55.0.xml

@@ -287,6 +287,17 @@
         </release-bug-list>
 
         <release-improvement-list>
+            <release-item>
+                <github-pull-request id="2601"/>
+
+                <release-item-contributor-list>
+                    <release-item-contributor id="david.steele"/>
+                    <release-item-reviewer id="stefan.fercot"/>
+                </release-item-contributor-list>
+
+                <p>Clarify incremental backup expiration.</p>
+            </release-item>
+
             <release-item>
                 <github-pull-request id="2541"/>
 

doc/xml/user-guide.xml

@@ -1852,7 +1852,7 @@
                 <backrest-config-option section="global" key="repo1-retention-diff">1</backrest-config-option>
             </backrest-config>
 
-            <p>Backup <br-setting>repo1-retention-diff=1</br-setting> so two differentials will need to be performed before one is expired. An incremental backup is added to demonstrate incremental expiration. Incremental backups cannot be expired independently &amp;mdash; they are always expired with their related full or differential backup.</p>
+            <p>Backup <br-setting>repo1-retention-diff=1</br-setting> so two differentials will need to be performed before one is expired. An incremental backup is added to demonstrate incremental expiration, which in this case depends on the differential expiration.</p>
 
             <execute-list host="{[host-pg1]}">
                 <title>Perform differential and incremental backups</title>