GH-3591: Add gateway() section into dsl.adoc

[artembilan]

Fixes #3591

[garyrussell]

Suggested change

	The `gateway()` operator of `IntegrationFlow` definition is a special service activator implementation to call some other endpoint or integration flow via their input channel and wait for reply from there.
	Technically it plays the same role as a nested `<gateway>` component for `<chain>` definition (see <<./chain.adoc#chain-gateway,Calling a Chain from within a Chain>>) and allows to make a code flow much cleaner and more straightforward.
	And logically and from business perspective it is a messaging gateway to let to distribute and reuse functionality between different parts of the target integration solution (see <<./gateway.adoc#gateway,Messaging Gateways>>).
	This operator has several overloads for different goals:
	- `gateway(String requestChannel)` to call some endpoint's input channel by its name;
	- `gateway(MessageChannel requestChannel)` to call some endpoint's input channel by its direct injection;
	- `gateway(IntegrationFlow flow)` to call the input channel of the provided `IntegrationFlow`.
	All of them have a variant with the second `Consumer<GatewayEndpointSpec>` argument to configure the target `GatewayMessageHandler` and respective `AbstractEndpoint`.
	Also the `IntegrationFlow`-based has a power that it allows to call existing `IntegrationFlow` bean or declare the flow as a sub-flow via an in-place lambda for an `IntegrationFlow` functional interface or have it extracted in a `private` method cleaner code style:
	The `gateway()` operator in an `IntegrationFlow` definition is a special service activator implementation, to call some other endpoint or integration flow via its input channel and wait for reply.
	Technically it plays the same role as a nested `<gateway>` component in a `<chain>` definition (see <<./chain.adoc#chain-gateway,Calling a Chain from within a Chain>>) and allows a flow to be cleaner and more straightforward.
	Logically, and from business perspective, it is a messaging gateway to allow the distribution and reuse of functionality between different parts of the target integration solution (see <<./gateway.adoc#gateway,Messaging Gateways>>).
	This operator has several overloads for different goals:
	- `gateway(String requestChannel)` to send a message to some endpoint's input channel by its name;
	- `gateway(MessageChannel requestChannel)` to send a message to some endpoint's input channel by its direct injection;
	- `gateway(IntegrationFlow flow)` to send a message to the input channel of the provided `IntegrationFlow`.
	All of these have a variant with the second `Consumer<GatewayEndpointSpec>` argument to configure the target `GatewayMessageHandler` and respective `AbstractEndpoint`.
	Also the `IntegrationFlow`-based methods allows calling existing `IntegrationFlow` bean or declare the flow as a sub-flow via an in-place lambda for an `IntegrationFlow` functional interface or have it extracted in a `private` method cleaner code style:
	IMPORTANT: If the downstream flow does not always return a reply, you should set the `requestTimeout` to 0 to prevent hanging the calling thread indefinitely.
	In that case, the flow will end at that point and the thread released for further work.