+docs some docs for leadership

Author: ktoso

Sources/DistributedActors/Cluster/Leadership.swift

@@ -223,9 +223,9 @@ extension Leadership {
     /// fulfilling this role whenever the minimum number of nodes exist. This may be useful when operation would
     /// potentially be unsafe given less than `minimumNrOfMembers` nodes.
     ///
-    // TODO: In situations which need strong guarantees, this leadership election scheme does NOT provide strong enough
-    /// guarantees, and you should consider using another scheme or consensus based modes.
     public struct LowestReachableMember: LeaderElection {
+        // TODO: In situations which need strong guarantees, this leadership election scheme does NOT provide strong enough
+        // guarantees, and you should consider using another scheme or consensus based modes.
         let minimumNumberOfMembersToDecide: Int
         let loseLeadershipIfBelowMinNrOfMembers: Bool
 
@@ -318,6 +318,7 @@ extension Leadership {
 // MARK: Leadership settings
 
 extension ClusterSystemSettings {
+    /// Configure leadership election using which the cluster leader should be decided.
     public struct LeadershipSelectionSettings {
         private enum _LeadershipSelectionSettings {
             case none

Sources/DistributedActors/Docs.docc/Clustering.md

@@ -180,7 +180,14 @@ The ``Cluster/Membership`` also offers a number of useful APIs to inspect the me
 
 ## Cluster Leadership
 
-TODO: document leadership and Leadership changes.
+The cluster has a few operations which must be performed in a consistent fashion, such as moving a joining member to the ``Cluster/MemberStatus/up`` state. Other member status changes such as becoming `joining` or `down` do not require such strict decision-making and are disseminated throughout the cluster even without a leader.
+
+Which cluster member is designated a leader is decided by the configured ``LeaderElection`` strategy.
+
+By default, the leader is selected in a coordination free method which relies on the membership state and member ordering. 
+For details, refer to the ``Leadership/LowestReachableMember`` documentation.
+
+You can configure leader election by changing the ``ClusterSystemSettings/autoLeaderElection`` setting while initializing your ``ClusterSystem``.
 
 ## Customizing Remote Calls
 

Sources/DistributedActors/Docs.docc/LeadershipSelectionSettings.md

@@ -0,0 +1,12 @@
+# ``DistributedActors/ClusterSystemSettings/LeadershipSelectionSettings``
+
+@Metadata {
+   @DocumentationExtension(mergeBehavior: append)
+}
+
+## Topics
+
+### Pre-defined leadership election strategies
+
+- ``lowestReachable(minNumberOfMembers:)``
+- ``none``