Add more doxygen comments for llvm::ThreadLocal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109683 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/System/ThreadLocal.h

@@ -19,6 +19,8 @@
 
 namespace llvm {
   namespace sys {
+    // ThreadLocalImpl - Common base class of all ThreadLocal instantiations.
+    // YOU SHOULD NEVER USE THIS DIRECTLY.
     class ThreadLocalImpl {
       void* data;
     public:
@@ -28,11 +30,18 @@ namespace llvm {
       const void* getInstance();
     };
     
+    /// ThreadLocal - A class used to abstract thread-local storage.  It holds,
+    /// for each thread, a pointer a single object of type T.
     template<class T>
     class ThreadLocal : public ThreadLocalImpl {
     public:
       ThreadLocal() : ThreadLocalImpl() { }
+      
+      /// get - Fetches a pointer to the object associated with the current
+      /// thread.  If no object has yet been associated, it returns NULL;
       T* get() { return static_cast<T*>(getInstance()); }
+      
+      // set - Associates a pointer to an object with the current thread.
       void set(T* d) { setInstance(d); }
     };
   }