[Analyzer] Improve documentation for unlock scenario

Also set default `pods_to_unlock` value to empty array

lib/cocoapods/installer/analyzer/locking_dependency_analyzer.rb

@@ -13,11 +13,23 @@ module Pod
         # is in update mode, to prevent it from upgrading the Pods that weren't
         # changed in the {Podfile}.
         #
+        # @param [Lockfile] lockfile the lockfile containing dependency constraints
+        #
+        # @param [Array<String>] pods_to_update
+        #        List of pod names which needs to be updated because installer is
+        #        in update mode for these pods. Pods in this list and all their recursive dependencies
+        #        will not be included in generated dependency graph
+        #
+        # @param [Array<String>] pods_to_unlock
+        #        List of pod names whose version constraints will be removed from  the generated dependency graph.
+        #        Recursive dependencies of the pods won't be affected. This is currently used to force local pods
+        #        to be evaluated again whenever checksum of the specification of the local pods changes.
+        #
         # @return [Molinillo::DependencyGraph<Dependency>] the dependencies
         #         generated by the lockfile that prevent the resolver to update
         #         a Pod.
         #
-        def self.generate_version_locking_dependencies(lockfile, pods_to_update, pods_to_unlock = nil)
+        def self.generate_version_locking_dependencies(lockfile, pods_to_update, pods_to_unlock = [])
           dependency_graph = Molinillo::DependencyGraph.new
 
           if lockfile