refs #5404, add a comment explaining what the test does

git-svn-id: file:///svn/toku/tokudb@47327 c7de825b-a66e-492c-adef-691d508d4ae1

src/tests/perf_txn_single_thread.cc

@@ -15,8 +15,8 @@
 
 #include "threaded_stress_test_helpers.h"
 
-// The intent of this test is to measure the throughput of the test infrastructure executing a nop
-// on multiple threads.
+// The intent of this test is to measure how fast a single thread can 
+// commit and create transactions when there exist N transactions.
 
 DB_TXN** txns;
 int num_txns;