INT-3555: Fix Header Channel Mapping

JIRA: https://jira.spring.io/browse/INT-3555

Previously, `replyChannel` and `errorChannel` headers were
unconditionally dropped in `AbstractHeaderMapper`. This was
changed in 4.1.0.M1 to allow these headers to be transmitted
in the case that they were translated to a String by a
`HeaderChannelRegistry`.

However, we should still drop these headers if they reference live
`MessageChannel` instances.
INT-3556: Payload Conversion Documentation

JIRA: https://jira.spring.io/browse/INT-3556

Clarify when a `GenericConverter` might be needed rather
than a simple `Converter`.