Add doc note for -server flag on Windows service

The -server flag was previously filtered by the Windows service
installation routine. Since the default jvm.options file no longer
includes this flag, this filtering was removed from the Windows
service. As this flag is not compatible with the Windows service, this
means that if a user has an existing jvm.options file that includes this
flag they will run into problems when they try to start the
Elasticsearch service on Windows. This commit adds a note to the
migration docs regarding this change.

Author: jasontedor

docs/reference/migration/migrate_6_0/packaging.asciidoc

@@ -61,6 +61,17 @@ We previously attempted to ensure that Elasticsearch could be started on 32-bit
 JVM (although a bootstrap check prevented using a 32-bit JVM in production). We
 are no longer maintaining this attempt.
 
+==== `-server` flag no longer filtered in the Windows service
+
+Related to the previous change that 32-bit is no longer supported, the default
+`jvm.options` file that ships with Elasticsearch no longer includes the
+`-server` flag. Previously, the installation of the Windows service would filter
+the `-server` flag because it is not compatible with the service. Since the
+default `jvm.options` file no longer includes `-server`, the filtering of this
+flag was removed from the installation of the Windows service. This means that
+if you have a `jvm.options` file that includes this flag you will need to remove
+it.
+
 ==== `ES_JVM_OPTIONS` is no longer supported
 
 The environment variable `ES_JVM_OPTIONS` that enabled a custom location for the