Break up long sentence; fix punctuation

Fixes: rdar://115730302

Author: amartini51

stdlib/public/Concurrency/ExecutorAssertions.swift

@@ -81,10 +81,10 @@ extension Actor {
   /// * In `-O` builds (the default for Xcode's Release configuration), stops
   ///   program execution.
   ///
-  /// - Note: This check is performed against the actor's serial executor,
-  ///   meaning that / if another actor uses the same serial executor--by using
+  /// - Note: Because this check is performed against the actor's serial executor,
+  ///   if another actor uses the same serial executor--by using
   ///   that actor's serial executor as its own ``Actor/unownedExecutor``--this
-  ///   check will succeed , as from a concurrency safety perspective, the
+  ///   check will succeed.  From a concurrency safety perspective, the
   ///   serial executor guarantees mutual exclusion of those two actors.
   ///
   /// - Parameters:

stdlib/public/Distributed/DistributedAssertions.swift

@@ -30,10 +30,10 @@ extension DistributedActor {
   /// * In `-O` builds (the default for Xcode's Release configuration), stops
   ///   program execution.
   ///
-  /// - Note: This check is performed against the actor's serial executor,
-  ///   meaning that / if another actor uses the same serial executor--by using
+  /// - Note: Because this check is performed against the actor's serial executor,
+  ///   if another actor uses the same serial executor--by using
   ///   that actor's serial executor as its own ``Actor/unownedExecutor``--this
-  ///   check will succeed , as from a concurrency safety perspective, the
+  ///   check will succeed.  From a concurrency safety perspective, the
   ///   serial executor guarantees mutual exclusion of those two actors.
   ///
   /// - Parameters: