[AL-1328] Added documentation for search channel implementations.
[AL-1328] Added documentation for search channel implementations.

uhef/Oskari-Routing-frontend,uhef/Oskari-Routing-frontend,uhef/Oskari-Routing-frontend,uhef/Oskari-Routing-frontend,uhef/Oskari-Routing-frontend,uhef/Oskari-Routing-frontend