Javadoc reactor.convert package in reactor-core

Add missing javadoc to the types in reactor.convert.

During this work, a bug was found in ConstructorParameterConverter.
When performing the conversion it was incorrectly looking for a
constructor on the source type, rather than on the target. This has
been corrected and a few unit tests have been added to verify the
behaviour.
Javadoc reactor.core.configuration package
Javadoc reactor.core.dynamic in reactor-core