Document about item action plugin ordering.

Signed-off-by: Raghuram Devarakonda <draghuram@gmail.com>

site/content/docs/main/custom-plugins.md

@@ -59,6 +59,9 @@ Velero supports the following kinds of plugins:
 - **Restore Item Action** - executes arbitrary logic for individual items prior to restoring them into a cluster
 - **Delete Item Action** - executes arbitrary logic based on individual items within a backup prior to deleting the backup
 
+Plugins are discovered by recursively reading a directory in no particular order. Hence no guarantee is provided for the
+order in which item action plugins are invoked.
+
 ## Plugin Logging
 
 Velero provides a [logger][2] that can be used by plugins to log structured information to the main Velero server log or