doc: update rocksDB tuning

[jjyr]

1. Remove the block size, use default value(4K). Tuning this value do not affect the performance of our program.
2. Increase write_buffer_size to 64M, in stress testing we see the error:

Transaction could not check for conflicts for operation at SequenceNumber 11871223343 as the MemTable only contains changes newer than SequenceNumber 11871336085.  Increasing the value of the max_write_buffer_size_to_maintain option could reduce the frequency of this error.

The max_write_buffer_size_to_maintain is setting to -1, so RocksDB will tuning max write buffer to write_buffer_size * max_write_buffer_number.

We increase the write_buffer_size to avoid the error.