Add some quick comments about folly::fiber implementation classes.

Summary: Add some quick comments about folly::fiber implementation classes.

Reviewed By: yfeldblum

Differential Revision: D2891940

fb-gh-sync-id: b9899bb63df03f32763f93a5dae8dff94c8e354e

folly/experimental/fibers/GuardPageAllocator.cpp

@@ -47,6 +47,8 @@ constexpr size_t kMaxInUse = 100;
 
 /**
  * A cache for kNumGuarded stacks of a given size
+ *
+ * Thread safe.
  */
 class StackCache {
  public:
@@ -176,6 +178,10 @@ class CacheManager {
   }
 };
 
+/*
+ * RAII Wrapper around a StackCache that calls
+ * CacheManager::giveBack() on destruction.
+ */
 class StackCacheEntry {
  public:
   explicit StackCacheEntry(size_t stackSize)