JIRA: https://jira.spring.io/browse/INT-4434
There were a restriction introduced since Java DSL `1.2` do not use
`IntegrationFlow` beans for sub-flow definitions, e.g. in routers.
It is considered as regression by community because it worked before
in version `1.1`
* Introduce `IntegrationFlow.getInputChannel()` to be able to bridge
from the main flow to the flow which is treated as sub-flow.
In most cases we talk about an independent bean for the `IntegrationFlow`
which can be used as a stand along one and as a sub-flow in other flow
**Cherry-pick to 5.0.x**
Add JavaDocs to the `EndpointSpec.obtainInputChannelFromFlow()`
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