[receiver/kubeletstats] Add clarifications on serviceAccount auth type (open-telemetry#31280)

**Description:** <Describe what has changed.>
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This PR adds some clarifications in the documentation of the
`serviceAccount` auth type of the `kubeletstats` receiver.
This is an outcome of the discussions at
open-telemetry/opentelemetry-helm-charts#1034 (comment).

Signed-off-by: ChrsMark <[email protected]>

Author: ChrsMark

receiver/kubeletstatsreceiver/README.md

@@ -37,7 +37,9 @@ endpoint will be used if `auth_type` set to any of the following values:
 - `tls` tells the receiver to use TLS for auth and requires that the fields
 `ca_file`, `key_file`, and `cert_file` also be set.
 - `serviceAccount` tells this receiver to use the default service account token
-to authenticate to the kubelet API.
+to authenticate to the kubelet API along with the default certificate which is signed by the cluster's root CA cert:
+  - `/var/run/secrets/kubernetes.io/serviceaccount/token`
+  - `/var/run/secrets/kubernetes.io/serviceaccount/ca.crt`
 - `kubeConfig` tells this receiver to use the kubeconfig file (KUBECONFIG env variable or ~/.kube/config)
 to authenticate and use API server proxy to access the kubelet API.
 - `initial_delay` (default = `1s`): defines how long this receiver waits before starting.