32 characters requirement for xpack.reporting.encryptionKey (elastic#…

…72593) (elastic#73472)

Similar to https://www.elastic.co/guide/en/kibana/current/alert-action-settings-kb.html#general-alert-action-settings is a 32 character minimum length required for xpack.reporting.encryptionKey

Co-authored-by: Toby Sutor <55087308+toby-sutor@users.noreply.github.com>

docs/settings/reporting-settings.asciidoc

@@ -21,7 +21,7 @@ You can configure `xpack.reporting` settings in your `kibana.yml` to:
   | Set to `false` to disable the {report-features}.
 
 | `xpack.reporting.encryptionKey`
-  | Set to any text string. By default, {kib} will generate a random key when it
+  | Set to an alphanumeric, at least 32 characters long text string. By default, {kib} will generate a random key when it
   starts, which will cause pending reports to fail after restart. Configure this
   setting to preserve the same key across multiple restarts and multiple instances of {kib}.