docs: update docs for allowed propagating apis flag (#660)

Co-authored-by: Vasudev Bongale <[email protected]>

docs/api-references.md

@@ -228,7 +228,7 @@ _Appears in:_
 
 
 ClusterResourcePlacement is used to select cluster scoped resources, including built-in resources and custom resources, and placement them onto selected member clusters in a fleet. 
- If a namespace is selected, ALL the resources under the namespace are placed to the target clusters. Note that you can't select the following resources: - reserved namespaces including: default, kube-* (reserved for Kubernetes system namespaces), fleet-* (reserved for fleet system namespaces). - reserved fleet resource types including: MemberCluster, InternalMemberCluster, ClusterResourcePlacement, ClusterSchedulingPolicySnapshot, ClusterResourceSnapshot, ClusterResourceBinding, etc. 
+ If a namespace is selected, ALL the resources under the namespace are placed to the target clusters unless `allowed-propagating-apis` flag is configured on hub-agent. Note that you can't select the following resources: - reserved namespaces including: default, kube-* (reserved for Kubernetes system namespaces), fleet-* (reserved for fleet system namespaces). - reserved fleet resource types including: MemberCluster, InternalMemberCluster, ClusterResourcePlacement, ClusterSchedulingPolicySnapshot, ClusterResourceSnapshot, ClusterResourceBinding, etc.
  `ClusterSchedulingPolicySnapshot` and `ClusterResourceSnapshot` objects are created when there are changes in the system to keep the history of the changes affecting a `ClusterResourcePlacement`.
 
 _Appears in:_

docs/faq.md

@@ -36,6 +36,8 @@ The fleet reserved namespace are `fleet-system` and `fleet-member-{clusterName}`
 
 ## What kind of the resources are allowed to be propagated from the hub cluster to the member clusters? How can I control the list?
 
+The resources to be propagated from the hub cluster to the member clusters can be controlled by either an exclude/skip list or an include/allow list which are mutually exclusive.
+
 `ClusterResourcePlacement` excludes certain groups/resources when propagating the resources by default. They are defined [here](https://github.com/Azure/fleet/blob/main/pkg/utils/apiresources.go).
 - `k8s.io/api/events/v1` (group)
 - `k8s.io/api/coordination/v1` (group)
@@ -46,6 +48,8 @@ The fleet reserved namespace are `fleet-system` and `fleet-member-{clusterName}`
 
 You can use `skipped-propagating-apis` and `skipped-propagating-namespaces` flag when installing the hub-agent to skip resources from being propagated by specifying their group/group-version/group-version-kind and namespaces.
 
+You can use `allowed-propagating-apis` flag on the hub-agent to only allow propagation of desired set of resources specified in the form of group/group-version/group-version-kind. This flag is mutually exclusive with `skipped-propagating-apis`.
+
 ## What happens to existing resources in member clusters when their definitions conflict with the desired resources in the hub cluster?
 
 In case of a conflict, where a resource already exists on the member cluster, the apply operation fails when trying to propagate the same resource from the hub cluster.