Add known issue for 6.4 (#33174)

Regarding the inability to use PEM keys exported from PKCS#12
containers as discussed in #33168

Author: jkakavas

docs/reference/release-notes/6.4.asciidoc

@@ -37,6 +37,20 @@
 [[release-notes-6.4.0]]
 == {es} version 6.4.0
 
+[float]
+=== Known issues
+
+{es} 6.4.0 fails to start when PEM encoded private key files that have been exported from `PKCS#12`
+keystores are in use. These files can be identified by the existence of lines that start with either
+`Bag Attributes` or `Key Attributes` in the beginning of the key file.
+
+These keys can be converted to a supported format using one of the following methods:
+
+* Remove the extra lines from the beginning of the file so that the file starts with the line that starts
+  with `-----BEGIN`
+* Use openssl to convert it, i.e: `openssl pkcs8 -in old_format.key -topk8 -nocrypt -out new_format.key`
+  assuming the key was not password protected.
+
 Also see <<breaking-changes-6.4>>.
 
 [float]