Update GPBMessage.m

Fix up some spelling/grammar.

objectivec/GPBMessage.m

@@ -80,18 +80,17 @@ static NSString *const kGPBDataCoderKey = @"GPBData";
   GPBFieldDescriptor *autocreatorField_;
   GPBExtensionDescriptor *autocreatorExtension_;
 
-  // Message can only be mutated from one thread. But some *readonly* operations modify internal
+  // Messages can only be mutated from one thread. But some *readonly* operations modify internal
   // state because they autocreate things. The autocreatedExtensionMap_ is one such structure.
-  // Access during readonly operations is protected via this semaphore.
+  // Access during readonly operations is protected via this lock.
   //
-  // Long ago, this was a OSSpinLock, but then it came to light that there were issues for that on
+  // Long ago, this was an OSSpinLock, but then it came to light that there were issues for that on
   // iOS:
   //   http://mjtsai.com/blog/2015/12/16/osspinlock-is-unsafe/
   //   https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html
-  // So it was a dispatch_semaphore_t, but that has issues with priority inversion if developer code
-  // uses queue for different things. But the minOS versions are now high enough, os_unfair_lock and
-  // be used, and should provide all the support needed. For more information in the
-  // concurrence/locking space see:
+  // It was changed to a dispatch_semaphore_t, but that has potential for priority inversion issues.
+  // The minOS versions are now high enough that os_unfair_lock can be used, and should provide 
+  // all the support we need. For more information in the concurrency/locking space see:
   //   https://gist.github.com/tclementdev/6af616354912b0347cdf6db159c37057
   //   https://developer.apple.com/library/archive/documentation/Performance/Conceptual/EnergyGuide-iOS/PrioritizeWorkWithQoS.html
   //   https://developer.apple.com/videos/play/wwdc2017/706/