Fix request unit calculator references and description (#19692)

Author: kunga

ydb/core/tx/schemeshard/schemeshard_billing_helpers.cpp

@@ -40,14 +40,25 @@ TString TBillingStats::ToString() const {
 }
 
 ui64 TRUCalculator::ReadTable(ui64 bytes) {
+    // The ReadTable operation lets you efficiently read large ranges of data from a table.
+    // The request cost only depends on the amount of data read based on the rate of 128 RU per 1 MB.
+    // When calculating the cost, the amount is rounded up to a multiple of 1 MB.
+    // https://yandex.cloud/en-ru/docs/ydb/pricing/ru-special#readtable
     return 128 * ((bytes + 1_MB - 1) / 1_MB);
 }
 
 ui64 TRUCalculator::BulkUpsert(ui64 bytes, ui64 rows) {
+    // BulkUpsert lets you efficiently upload data to the database.
+    // The cost of writing a row using the BulkUpsert operation is 0.5 RU per 1 KB of written data.
+    // When calculating the cost, the data amount is rounded up to a multiple of 1 KB.
+    // The total cost of the operation is calculated as the sum of costs for all rows written, with the result rounded up to the nearest integer.
+    // https://yandex.cloud/en-ru/docs/ydb/pricing/ru-special#bulkupsert
     return (Max(rows, (bytes + 1_KB - 1) / 1_KB) + 1) / 2;
 }
 
 ui64 TRUCalculator::Calculate(const TBillingStats& stats) {
+    // The cost of building an index is the sum of the cost of ReadTable from the source table and BulkUpsert to the index table.
+    // https://yandex.cloud/en-ru/docs/ydb/pricing/ru-special#secondary-index
     return TRUCalculator::ReadTable(stats.GetReadBytes())
          + TRUCalculator::BulkUpsert(stats.GetUploadBytes(), stats.GetUploadRows());
 }

ydb/core/tx/schemeshard/schemeshard_billing_helpers.h

@@ -52,7 +52,6 @@ class TBillingStats {
 };
 
 struct TRUCalculator {
-    // https://a.yandex-team.ru/arc/trunk/arcadia/kikimr/docs/ru/pricing/serverless.md
     static ui64 ReadTable(ui64 bytes);
     static ui64 BulkUpsert(ui64 bytes, ui64 rows);
     static ui64 Calculate(const TBillingStats& stats);