message: core: remove misleading dispatcher docs
  There's no automated propagation of subscribers between spans, you have
  to do this manually.
diff: @@ -5,15 +5,13 @@ pub use tracing_core::collect::*;
  #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
  pub use tracing_core::dispatch::DefaultGuard;
  -/// Sets this collector as the default for the duration of a closure.
  +/// Sets this collector as the default for the current thread for the duration
  +/// of a closure.
  ///
  /// The default collector is used when creating a new [`Span`] or
  -/// [`Event`], _if no span is currently executing_. If a span is currently
  -/// executing, new spans or events are dispatched to the collector that
  -/// tagged that span, instead.
  +/// [`Event`].
  ///
  /// [`Span`]: super::span::Span
  -/// [`Collect`]: super::collect::Collect
  /// [`Event`]: tracing_core::Event
  #[cfg(feature = "std")]
  #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
  @@ -43,13 +41,11 @@ where
  crate::dispatch::set_global_default(crate::Dispatch::new(collector))
  }
  -/// Sets the collector as the default for the duration of the lifetime of the
  -/// returned [`DefaultGuard`]
  +/// Sets the collector as the default for the current thread for the duration of
  +/// the lifetime of the returned [`DefaultGuard`].
  ///
  /// The default collector is used when creating a new [`Span`] or
  -/// [`Event`], _if no span is currently executing_. If a span is currently
  -/// executing, new spans or events are dispatched to the collector that
  -/// tagged that span, instead.
  +/// [`Event`].
  ///
  /// [`Span`]: super::span::Span
  /// [`Event`]: tracing_core::Event
  