Clarify SELECT description

1. At the moment select documentation contains Запрос SELECT используется для получения информации из указанной таблицы в базе данных. It is not correct as SELECT returns projection from a relation, that can be build from arbitrary amount of tables (via JOIN for example) - #416 (closed).
2. Give an example of explain select ... plan and describe its internals.
   • Need to introduce SELECTION, PROJECTION and SCAN plan nodes (look at presentation from #412 (moved)). Keep in mind that PROJECTION must always be above SELECTION (optional).
   • Need examples for each possible combination:
     • SCAN + PROJECTION: select a from t, select * from t
     • SCAN + SELECTION + PROJECTION: select a from t where a = 1
   • Explain what distribution key means and how it helps us to reduce the amount of instances to query (part of #418 (moved))