Clarify documentation in folly::coro::Task about when RequestContext is captured

Summary:
Clarify that the RequestContext is captured at the time the coroutine is
started (e.g. due to co_awaiting it or calling .start()) rather than at
the time the coroutine is called.

Reviewed By: andriigrynenko

Differential Revision: D21353947

fbshipit-source-id: 40ec7b25d84e96a611a1f63b6184a0d6a016948d

folly/experimental/coro/Task.h

@@ -453,9 +453,10 @@ class FOLLY_NODISCARD TaskWithExecutor {
 /// always resumes on the executor.
 ///
 /// The Task coroutine is RequestContext-aware and will capture the
-/// current RequestContext at the time the coroutine function is called and
-/// will save/restore the current RequestContext whenever the coroutine
-/// suspends and resumes at a co_await expression.
+/// current RequestContext at the time the coroutine function is either
+/// awaited or explicitly started and will save/restore the current
+/// RequestContext whenever the coroutine suspends and resumes at a co_await
+/// expression.
 template <typename T>
 class FOLLY_NODISCARD Task {
  public: