Add a comment about the usage of GTM_CountOpenTransactions
Add a comment about the usage of GTM_CountOpenTransactions

This macro should be used with READ lock hold on gt_TransArrayLock when
called and it is let to the user to do the lock call himself.

Per patch from Nikhil Deshpande

Postgres-XL/Postgres-XL,pavanvd/postgres-xl,oberstet/postgres-xl,arcivanov/postgres-xl,postmind-net/postgres-xl,kmjungersen/PostgresXL,zeroae/postgres-xl,snaga/postgres-xl,zeroae/postgres-xl,arcivanov/postgres-xl,yazun/postgres-xl,pavanvd/postgres-xl,postmind-net/postgres-xl,oberstet/postgres-xl,techdragon/Postgres-XL,ovr/postgres-xl,pavanvd/postgres-xl,arcivanov/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,kmjungersen/PostgresXL,kmjungersen/PostgresXL,arcivanov/postgres-xl,ovr/postgres-xl,techdragon/Postgres-XL,yazun/postgres-xl,snaga/postgres-xl,yazun/postgres-xl,kmjungersen/PostgresXL,postmind-net/postgres-xl,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,zeroae/postgres-xl,techdragon/Postgres-XL,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,zeroae/postgres-xl,pavanvd/postgres-xl,snaga/postgres-xl,oberstet/postgres-xl,yazun/postgres-xl,kmjungersen/PostgresXL,techdragon/Postgres-XL,yazun/postgres-xl,postmind-net/postgres-xl,zeroae/postgres-xl,oberstet/postgres-xl,postmind-net/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,snaga/postgres-xl,ovr/postgres-xl,pavanvd/postgres-xl,snaga/postgres-xl,oberstet/postgres-xl