search: improve API docs & remove field ambiguity

Created by: slimsag

This PR improves documentation of the GraphQL search API by:

1. Clarifying that resultCount is counting the number of returned results, NOT the number of overall results (i.e. if count:1000 and there are more than 1,000 results, resultCount would be 1000 and not the total number of results the search theoretically has if a higher count parameter was specified).
2. Renaming resultCount to matchCount to remove the (currently documented) ambiguity that exists between "what is the difference between resultCount and len(results)? When should I use one or the other?"

Test plan: existing tests / not needed