[Fleet] Added readme about bumping agent policies on SO change (elast…

…ic#201742)

## Summary

Resolves elastic#193352

Documented the usage of `bump_agent_policy_revision` and using
`revision` to bump on agent policy SO changes.

I think it's not needed to introduce `bump_agent_policy_revision` on the
agent policy SO type, enough to bump revision, and the agent policy will
be deployed on Fleet setup when
https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/server/services/agent_policies/deploy_agent_policies_task.ts
is running.

---------

Co-authored-by: Cristina Amico <[email protected]>

x-pack/plugins/fleet/server/saved_objects/bump_agent_policies.md

@@ -0,0 +1,33 @@
+## Agent policy SO changes
+
+When making changes to agent policy SO, the changes are not automatically deployed to agents. To trigger an agent policy bump, add a new model version to the agent policy SO type with a revision increase.
+
+```
+        '2': {
+          changes: [
+            {
+              type: 'data_backfill',
+              backfillFn: (doc) => {
+                return { attributes: { ...doc.attributes, revision: doc.attributes.revision + 1 } };
+              },
+            },
+          ],
+        },
+```
+
+## Package policy SO changes
+
+Similarly, package policy SO changes do not automatically trigger a redeploy of agent policies using them. To trigger an agent policy bump using package policies, add a new model version to the package policy SO type with `bump_agent_policy_revision: true`.
+
+```
+   '2': {
+          changes: [
+            {
+              type: 'data_backfill',
+              backfillFn: (doc) => {
+                return { attributes: { ...doc.attributes, bump_agent_policy_revision: true } };
+              },
+            },
+          ],
+        },
+```