improve Publisher 1.3 reactive-streams#431

Author: davidmoten

README.md

@@ -98,8 +98,8 @@ public interface Publisher<T> {
 | [:bulb:](#1.1 "1.1 explained") | *The intent of this rule is to make it clear that Publishers cannot signal more elements than Subscribers have requested. There’s an implicit, but important, consequence to this rule: Since demand can only be fulfilled after it has been received, there’s a happens-before relationship between requesting elements and receiving elements.* |
 | <a name="1.2">2</a>       | A `Publisher` MAY signal fewer `onNext` than requested and terminate the `Subscription` by calling `onComplete` or `onError`. |
 | [:bulb:](#1.2 "1.2 explained") | *The intent of this rule is to make it clear that a Publisher cannot guarantee that it will be able to produce the number of elements requested; it simply might not be able to produce them all; it may be in a failed state; it may be empty or otherwise already completed.* |
-| <a name="1.3">3</a>       | `onSubscribe`, `onNext`, `onError` and `onComplete` signaled to a `Subscriber` MUST be signaled in a [thread-safe](#term_thread-safe) manner—and if performed by multiple threads—use [external synchronization](#term_ext_sync). |
-| [:bulb:](#1.3 "1.3 explained") | *The intent of this rule is to make it clear that [external synchronization](#term_ext_sync) must be employed if the Publisher intends to send signals from multiple/different threads.* |
+| <a name="1.3">3</a>       | `onSubscribe`, `onNext`, `onError` and `onComplete` signaled to a `Subscriber` MUST be signaled serially. |
+| [:bulb:](#1.3 "1.3 explained") | *The intent of this rule is to permit the signalling of signals from multiple threads if and only if a happens-before relation between each of the signals is established.* |
 | <a name="1.4">4</a>       | If a `Publisher` fails it MUST signal an `onError`. |
 | [:bulb:](#1.4 "1.4 explained") | *The intent of this rule is to make it clear that a Publisher is responsible for notifying its Subscribers if it detects that it cannot proceed—Subscribers must be given a chance to clean up resources or otherwise deal with the Publisher´s failures.* |
 | <a name="1.5">5</a>       | If a `Publisher` terminates successfully (finite stream) it MUST signal an `onComplete`. |