fix documentation about low level transaction API usage #389

According to the encode/Databases documentation about Transactions, a common pattern for lower-level transaction API is:

transaction = await database.transaction()
try:
    await transaction.start()
    ...
except:
    await transaction.rollback()
else:
    await transaction.commit()

However, calling await transaction.start() is a duplication and leads to error because it is implicitly invoked by await database.transaction(), as can be seen from this snippet of code.

So, the documentation should skip calling start:

transaction = await database.transaction()
try:
    ...
except:
    await transaction.rollback()
else:
    await transaction.commit()

In the chat room, @aminalaee pointed out it would be possible to use start too if the transaction instantiation wasn't awaited:

transaction = database.transaction()
await transaction.start()
try:
    ...
except:
    await transaction.rollback()
else:
    await transaction.commit()

Although both forms are correct, keeping them both in the documentation will be confusing. I recommend that we keep the first one because it is simpler.