provides clarification on the 1.9 rule

Author: OlegDokuka

README.md

@@ -110,8 +110,8 @@ public interface Publisher<T> {
 | [:bulb:](#1.7 "1.7 explained") | *The intent of this rule is to make sure that onError and onComplete are the final states of an interaction between a Publisher and Subscriber pair.* |
 | <a name="1.8">8</a>       | If a `Subscription` is cancelled its `Subscriber` MUST eventually stop being signaled. |
 | [:bulb:](#1.8 "1.8 explained") | *The intent of this rule is to make sure that Publishers respect a Subscriber’s request to cancel a Subscription when Subscription.cancel() has been called. The reason for **eventually** is because signals can have propagation delay due to being asynchronous.* |
-| <a name="1.9">9</a>       | `Publisher.subscribe` MUST call `onSubscribe` on the provided `Subscriber` prior to any other signals to that `Subscriber` and MUST [return normally](#term_return_normally), except when the provided `Subscriber` is `null` in which case it MUST throw a `java.lang.NullPointerException` to the caller, for all other situations the only legal way to signal failure (or reject the `Subscriber`) is by calling `onError` (after calling `onSubscribe`). |
-| [:bulb:](#1.9 "1.9 explained") | *The intent of this rule is to make sure that `onSubscribe` is always signalled before any of the other signals, so that initialization logic can be executed by the Subscriber when the signal is received. Also `onSubscribe` MUST only be called at most once, [see [2.12](#2.12)]. If the supplied `Subscriber` is `null`, there is nowhere else to signal this but to the caller, which means a `java.lang.NullPointerException` must be thrown. Examples of possible situations: A stateful Publisher can be overwhelmed, bounded by a finite number of underlying resources, exhausted, or in a [terminal state](#term_terminal_state).* |
+| <a name="1.9">9</a>       | `Publisher.subscribe` MUST call `onSubscribe` [synchronously](#term_sync) on the provided `Subscriber`, and do so prior to any other signals to that `Subscriber`, and MUST [return normally](#term_return_normally), except when the provided `Subscriber` is `null` in which case it MUST throw a `java.lang.NullPointerException` to the caller, for all other situations the only legal way to signal failure (or reject the `Subscriber`) is by calling `onError` (after calling `onSubscribe`). |
+| [:bulb:](#1.9 "1.9 explained") | *The intent of this rule is to make sure that `onSubscribe` is always signalled before any of the other signals and is called synchronusly with in the `subscribe` method, so that initialization logic can be executed by the Subscriber when the signal is received. Also `onSubscribe` MUST only be called at most once, [see [2.12](#2.12)]. If the supplied `Subscriber` is `null`, there is nowhere else to signal this but to the caller, which means a `java.lang.NullPointerException` must be thrown. Examples of possible situations: A stateful Publisher can be overwhelmed, bounded by a finite number of underlying resources, exhausted, or in a [terminal state](#term_terminal_state).* |
 | <a name="1.10">10</a>     | `Publisher.subscribe` MAY be called as many times as wanted but MUST be with a different `Subscriber` each time [see [2.12](#2.12)]. |
 | [:bulb:](#1.10 "1.10 explained") | *The intent of this rule is to have callers of `subscribe` be aware that a generic Publisher and a generic Subscriber cannot be assumed to support being attached multiple times. Furthermore, it also mandates that the semantics of `subscribe` must be upheld no matter how many times it is called.* |
 | <a name="1.11">11</a>     | A `Publisher` MAY support multiple `Subscriber`s and decides whether each `Subscription` is unicast or multicast. |