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>

src/build/help/help.xml

@@ -917,6 +917,8 @@
 
                         <text>
                             <p>When a differential backup expires, all incremental backups associated with the differential backup will also expire. When not defined all differential backups will be kept until the full backups they depend on expire.</p>
+
+                            <p>Note that full backups are included in the count of differential backups for the purpose of expiration. This slightly reduces the number of differential backups that need to be retained in most cases.</p>
                         </text>
 
                         <example>3</example>
@@ -2227,7 +2229,9 @@
                 <summary>Expire backups that exceed retention.</summary>
 
                 <text>
-                    <p><backrest/> does full backup rotation based on the retention type which can be a count or a time period. When a count is specified, then expiration is not concerned with when the backups were created but with how many must be retained. Differential and Incremental backups are count-based but will always be expired when the backup they depend on is expired. See sections <link page="user-guide" section="/retention/full">Full Backup Retention</link> and <link page="user-guide" section="/retention/diff">Differential Backup Retention</link> for details and examples. Archived WAL is retained by default for backups that have not expired, however, although not recommended, this schedule can be modified per repository with the retention-archive options. See section <link page="user-guide" section="/retention/archive">Archive Retention</link> for details and examples.</p>
+                    <p><backrest/> does full backup rotation based on the retention type which can be a count or a time period. When a count is specified, then expiration is not concerned with when the backups were created but with how many must be retained. Differential backups are count-based but will always be expired when the full backup they depend on is expired. Incremental backups are not expired by retention independently &amp;mdash; they are always expired with their related full or differential backup. See sections <link page="user-guide" section="/retention/full">Full Backup Retention</link> and <link page="user-guide" section="/retention/diff">Differential Backup Retention</link> for details and examples.</p>
+
+                    <p>Archived WAL is retained by default for backups that have not expired, however, although not recommended, this schedule can be modified per repository with the retention-archive options. See section <link page="user-guide" section="/retention/archive">Archive Retention</link> for details and examples.</p>
 
                     <p>The <cmd>expire</cmd> command is run automatically after each successful backup and can also be run by the user. When run by the user, expiration will occur as defined by the retention settings for each configured repository. If the <br-option>{[dash]}-repo</br-option> option is provided, expiration will occur only on the specified repository. Expiration can also be limited by the user to a specific backup set with the <br-option>--set</br-option> option and, unless the <br-option>{[dash]}-repo</br-option> option is specified, all repositories will be searched and any matching the set criteria will be expired. It should be noted that the archive retention schedule will be checked and performed any time the <cmd>expire</cmd> command is run.</p>
                 </text>