message: Documentation for countDocuments MUST mention estimatedDocumentCount
  Update Doxygen text for count_documents() to mention
  estimated_document_count(), some users found it difficult to discover
  one from the other.
diff: @@ -413,6 +413,9 @@ class MONGOCXX_API collection {
  ///
  /// @throws mongocxx::query_exception if the count operation fails.
  ///
  + /// @note For a fast count of the total documents in a collection, see estimated_document_count().
  + /// @see mongocxx::estimated_document_count
  + ///
  std::int64_t count_documents(bsoncxx::document::view_or_value filter,
  const options::count& options = options::count());
  @@ -449,6 +452,8 @@ class MONGOCXX_API collection {
  ///
  /// @throws mongocxx::query_exception if the count operation fails.
  ///
  + /// @see mongocxx::count_documents
  + ///
  std::int64_t estimated_document_count(
  const options::estimated_document_count& options = options::estimated_document_count());
  ///
  