Document the index corruption bug that gets fixed via Lucene 8.5.1. (#55232)

Using soft deletes on shrunk indices may cause corruption.

Author: jpountz

docs/reference/release-notes/7.0.asciidoc

@@ -109,6 +109,14 @@ Also see <<breaking-changes-7.0,Breaking changes in 7.0>>.
 [float]
 === Known issues
 
+* Applying deletes or updates on an index after it has been shrunk may corrupt
+the index. In order to prevent this issue, it is recommended to stop shrinking
+read-write indices. For read-only indices, it is recommended to force-merge
+indices after shrinking, which significantly reduces the likeliness of this
+corruption in the case that deletes/updates would be applied by mistake. This
+bug is fixed in {es} 7.7 and later versions. More details can be found on the
+https://issues.apache.org/jira/browse/LUCENE-9300[corresponding issue].
+
 * Indices created in 6.x with <<date,`date`>> and <<date_nanos,`date_nanos`>> fields using formats
 that are incompatible with java.time patterns will cause parsing errors, incorrect date calculations or wrong search results.
 https://github.com/elastic/elasticsearch/pull/52555