Improve description for MTRDevice. (project-chip#34857)

* apply changes from PR project-chip#34797 to to concrete class

* Improve description for MTRDevice.

Includes various information that wasn't included before but probably should be.

* Address review comments.

---------

Co-authored-by: Kiel Oleson <kielo@apple.com>

Author: 2 people

src/darwin/Framework/CHIP/MTRAsyncWorkQueue.h

@@ -228,6 +228,9 @@ MTR_TESTABLE
 - (BOOL)hasDuplicateForTypeID:(NSUInteger)opaqueDuplicateTypeID
                  workItemData:(id)opaqueWorkItemData;
 
+// Returns current count of queued items.
+- (NSUInteger)itemCount;
+
 /// Cancels and removes all work items.
 - (void)invalidate;
 @end

src/darwin/Framework/CHIP/MTRAsyncWorkQueue.mm

@@ -433,6 +433,12 @@ - (BOOL)hasDuplicateForTypeID:(NSUInteger)opaqueDuplicateTypeID workItemData:(id
     return NO;
 }
 
+- (NSUInteger)itemCount
+{
+    std::lock_guard lock(_lock);
+    return _items.count;
+}
+
 @end
 
 NS_ASSUME_NONNULL_END