[doc] Improve Mono#share javadoc for cancelling

Similarly to Flux#share, Mono#share also cancels the source when all
Subscribers have cancelled. This change improves the documentation.

Following #2680 and #2756 there exists a misalignment in the javadoc for
Mono#share method. Since cancelling the source is a fact, the
javadoc is now improved instead of changing the behaviour.

Resolves #3740

reactor-core/src/main/java/reactor/core/publisher/Mono.java

@@ -4236,9 +4236,10 @@ public final Mono<T> retryWhen(Retry retrySpec) {
 	/**
 	 * Prepare a {@link Mono} which shares this {@link Mono} result similar to {@link Flux#shareNext()}.
 	 * This will effectively turn this {@link Mono} into a hot task when the first
-	 * {@link Subscriber} subscribes using {@link #subscribe()} API. Further {@link Subscriber} will share the same {@link Subscription}
+	 * {@link Subscriber} subscribes using {@link #subscribe()} API.
+	 * Further {@link Subscriber} will share the same {@link Subscription}
 	 * and therefore the same result.
-	 * It's worth noting this is an un-cancellable {@link Subscription}.
+	 * When all subscribers have cancelled it will cancel the source {@link Mono}.
 	 * <p>
 	 * <img class="marble" src="doc-files/marbles/shareForMono.svg" alt="">
 	 *