Update documentation

[jerrinsg]

Some documentation has become outdated with the new changes introduced to the transaction package. These need to be updated. Some things that could be changed:

• The example application in examples/database.go uses the library logging approach. We could also add a function that does updates through a txn{} block
• Documentation is missing about the new optimized Log() function. This function is also missing from transaction/README.md
• There have been concerns raised about the locking model of go-pmem, so it will good to explictly mention that in the README.