GH-1956 Add support for PollableSource in functional programing model

This feature effectively provides identical path to create PollableSource as it was in annotation-based programming model.
However, unlike creating and  providing binding intreface all that is required from user is 'spring.cloud.stream.pollable-source' property

Resolves #1956

docs/src/main/asciidoc/spring-cloud-stream.adoc

@@ -1242,21 +1242,18 @@ support.
 ===== Overview
 
 When using polled consumers, you poll the `PollableMessageSource` on demand.
-Consider the following example of a polled consumer:
+To define binding for polled consumer you need to provide `spring.cloud.stream.pollable-source` property.
 
-[source,java]
+Consider the following example of a polled consumer binding:
+
+[source,text]
 ----
-public interface PolledConsumer {
-
-    @Input
-    PollableMessageSource destIn();
-
-    @Output
-    MessageChannel destOut();
-
-}
+--spring.cloud.stream.pollable-source=myDestination
 ----
 
+The pollable-source name `myDestination` in the preceding example will result in `myDestination-in-0` binding name to stay 
+consistent with functional programming model.
+
 Given the polled consumer in the preceding example, you might use it as follows:
 
 [source,java]
@@ -1348,6 +1345,7 @@ boolean result = pollableSource.poll(received -> {
 		}, new ParameterizedTypeReference<Map<String, Foo>>() {});
 ----
 
+
 [[polled-errors]]
 ===== Handling Errors