WebSearch: richer `/record` and `/search` API docs

* Enriches documentation about `/search` API with new sections
  describing XML API and JSON API, with notes and usage examples.
  (addresses #2303)

* Adds new "Record API" documentation describing `/record/` API, with
  notes and usage examples.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>