Add docs for known-issue joda pattern failures in 7.1

Author: pgomulka

docs/reference/release-notes/7.0.asciidoc

@@ -105,6 +105,15 @@ releases of 7.0.0.
 
 Also see <<breaking-changes-7.0,Breaking changes in 7.0>>.
 
+[float]
+=== Known issues
+
+* 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
+This is fixed in {es} 7.7 and later versions.
+
+
 [[breaking-7.0.0]]
 [float]
 === Breaking changes

docs/reference/release-notes/7.1.asciidoc

@@ -3,6 +3,14 @@
 
 Also see <<breaking-changes-7.1,Breaking changes in 7.1>>.
 
+[float]
+=== Known issues
+
+* 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
+This is fixed in {es} 7.7 and later versions.
+
 [[bug-7.1.1]]
 [float]
 === Bug fixes