Fix msvc ProducerConsumerQueue compilation and update its doc

Summary:
Compilation of basic sample from doc using VS2017 currently fails with:
```
C2327 'folly::ProducerConsumerQueue<folly::fbstring>::writeIndex_': is not a type name, static, or enumerator
```
Fixed it and updated the docs.
Closes https://github.com/facebook/folly/pull/806

Reviewed By: akrieger

Differential Revision: D7379931

Pulled By: yfeldblum

fbshipit-source-id: 6b7a72abd3324af56de355a6e65bdc7e1195596d

folly/ProducerConsumerQueue.h

@@ -173,16 +173,16 @@ struct ProducerConsumerQueue {
   }
 
  private:
+  using AtomicIndex = std::atomic<unsigned int>;
+
   char pad0_[hardware_destructive_interference_size];
   const uint32_t size_;
   T* const records_;
 
-  alignas(hardware_destructive_interference_size)
-      std::atomic<unsigned int> readIndex_;
-  alignas(hardware_destructive_interference_size)
-      std::atomic<unsigned int> writeIndex_;
+  alignas(hardware_destructive_interference_size) AtomicIndex readIndex_;
+  alignas(hardware_destructive_interference_size) AtomicIndex writeIndex_;
 
-  char pad1_[hardware_destructive_interference_size - sizeof(writeIndex_)];
+  char pad1_[hardware_destructive_interference_size - sizeof(AtomicIndex)];
 };
 
 } // namespace folly

folly/docs/ProducerConsumerQueue.md

@@ -40,7 +40,7 @@ the queue filling  or staying empty for long is unlikely.
 A toy example that doesn't really do anything useful:
 
 ``` Cpp
-    folly::ProducerConsumerQueue<folly::fbstring> queue;
+    folly::ProducerConsumerQueue<folly::fbstring> queue{size};
 
     std::thread reader([&queue] {
       for (;;) {