KAFKA-9226: Updated documentation section on log deletion policies. (#7738)

Reviewers: Mickael Maison <mickael.maison@gmail.com>

docs/implementation.html

@@ -219,7 +219,15 @@
     </pre>
     <h4><a id="impl_deletes" href="#impl_deletes">Deletes</a></h4>
     <p>
-    Data is deleted one log segment at a time. The log manager allows pluggable delete policies to choose which files are eligible for deletion. The current policy deletes any log with a modification time of more than <i>N</i> days ago, though a policy which retained the last <i>N</i> GB could also be useful. To avoid locking reads while still allowing deletes that modify the segment list we use a copy-on-write style segment list implementation that provides consistent views to allow a binary search to proceed on an immutable static snapshot view of the log segments while deletes are progressing.
+    Data is deleted one log segment at a time. The log manager applies two metrics to identify segments which are
+        eligible for deletion: time and size. For time-based policies, the record timestamps are considered, with the
+        largest timestamp in a segment file (order of records is not relevant) defining the retention time for the
+        entire segment. Size-based retention is disabled by default. When enabled the log manager keeps deleting the
+        oldest segment file until the overall size of the partition is within the configured limit again. If both
+        policies are enabled at the same time, a segment that is eligible for deletion due to either policy will be
+        deleted. To avoid locking reads while still allowing deletes that modify the segment list we use a copy-on-write
+        style segment list implementation that provides consistent views to allow a binary search to proceed on an
+        immutable static snapshot view of the log segments while deletes are progressing.
     </p>
     <h4><a id="impl_guarantees" href="#impl_guarantees">Guarantees</a></h4>
     <p>