Add info on Hybrid Search Scoring

Currently, we do not have information on the scoring for hybrid search. 

From the engineering design docs:

The score is consolidated across the regular exact search and kNN search using the pre-existing mechanism for a compound query, which is -

hit_score = (query_1_boost * query_1_hit_score) + (query_2_boost * query_2_hit_score)

This has already affected our partners at AWS.