Firestore: Small Count API documentation improvements (#1511)

Author: dconeybe

firestore/src/include/firebase/firestore/aggregate_source.h

@@ -32,7 +32,7 @@ enum class AggregateSource {
  * considering any local state. That is, documents in the local cache are not
  * taken into consideration, neither are local modifications not yet
  * synchronized with the server. Previously-downloaded results, if any, are
- * not used: every request using this source necessarily involves a round trip
+ * not used. Every request using this source necessarily involves a round trip
  * to the server.
  *
  * The AggregateQuery will fail if the server cannot be reached, such as if

firestore/src/include/firebase/firestore/query.h

@@ -152,8 +152,8 @@ class Query {
  *
  * Using the returned query to count the documents is efficient because only
  * the final count, not the documents' data, is downloaded. The returned query
- * can even count the documents if the result set would be prohibitively large
- * to download entirely (e.g. thousands of documents).
+ * can count the documents in cases where the result set is prohibitively
+ * large to download entirely (thousands of documents).
  *
  * @return An aggregate query that counts the documents in the result set of
  * this query.