Document caveats for 2.x to 3.x loki helm chart upgrades (grafana#11199)

**What this PR does / why we need it**:
grafana#9427 discusses a few notable
gotchas and differences between `v2.x` and `v3.x` helm charts. In this
PR, I would like to propose aggregation of the information from the
mentioned issue and appending that to the upgrade guide.

**Which issue(s) this PR fixes**:
see also: grafana#9427

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [x] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`

---------

Signed-off-by: Jan Wozniak <wozniak.jan@gmail.com>
Co-authored-by: J Stickler <julie.stickler@grafana.com>
Co-authored-by: Michel Hollands <42814411+MichelHollands@users.noreply.github.com>

docs/sources/setup/upgrade/upgrade-from-2x/index.md

@@ -47,6 +47,28 @@ helm upgrade loki grafana/loki \
 
 You will need to manually delete the existing stateful set for the above command to work.
 
+#### Notable changes
+
+The `grafana/loki` chart used `Secret` as storage for configuration.  You can set `.loki.existingSecretForConfig` to continue using `Secret` or migrate your configuration to a `ConfigMap`. Specifying the Loki config in `values.yaml` is still available. In the old chart it was under `.config`, the new chart allows specifying either `.loki.config` or `.loki.structuredConfig` which takes precedence.
+
+Similarly when using `extraVolumes`, the configuration is now nested under `.singleBinary.extraVolumes` or `.read.extraVolumes` + `.write.extraVolumes` if you decide to migrate to the Loki scalable deployment mode.
+
+#### Dependencies
+
+The `grafana/loki` chart was only used to install Loki. New charts since `v3.x` also bundle two dependencies - **minio** and **grafana-agent-operator**. If you have already installed either of these independently and wish to continue managing them separately, you can explicitly disable these dependencies in your `values.yaml` as shown in the following examples:
+```yaml
+minio:
+  enabled: false
+```
+
+```yaml
+monitoring:
+  selfMonitoring:
+    enabled: false
+    grafanaAgent:
+      installOperator: false
+```
+
 ### Upgrading from `grafana/loki-simple-scalable`
 
 As this chart is largely based off the `grafana/loki-simple-scalable` chart, you should be able to use your existing `values.yaml` file and just upgrade to the new chart name. For example, if you installed the `grafana/loki-simple-scalable` chart as `loki` in the namespace `loki`, your upgrade would be: