Add docs on disable-controllers-or-webhooks

This commit adds some minor documentation on how to use the new flag,
disable-controllers-or-webhooks.

Signed-off-by: Bryan Cox <[email protected]>

Author: bryan-cox

docs/book/src/self-managed/externally-managed-azure-infrastructure.md

@@ -7,3 +7,13 @@ If the `AzureCluster` resource includes a "cluster.x-k8s.io/managed-by" annotati
 This is useful for scenarios where a different persona is managing the cluster infrastructure out-of-band while still wanting to use CAPI for automated machine management.
 
 You should only use this feature if your cluster infrastructure lifecycle management has constraints that the reference implementation does not support. See [user stories](https://github.com/kubernetes-sigs/cluster-api/blob/10d89ceca938e4d3d94a1d1c2b60515bcdf39829/docs/proposals/20210203-externally-managed-cluster-infrastructure.md#user-stories) for more details. 
+
+## Disabling Specific Component Reconciliation
+Some controllers/webhooks may not be necessary to run in an externally managed cluster infrastructure scenario. These 
+controllers/webhooks can be disabled through a flag on the manager called `disable-controllers-or-webhooks`. This flag 
+accepts a comma separated list of values.
+
+Currently, these are the only accepted values:
+1. `DisableASOSecretController` - disables the ASOSecretController from being deployed
+2. `DisableAzureJSONMachineController` - disables the AzureJSONMachineController from being deployed
+