Fix several mismatching default option values and their comments

[aleksraiden]

In this PR:

• rocksdb.max_open_files defaults setup to 8096, but in comment old value - 4096
• Correct notice about max_write_buffer_number behavior (Misspelling, must be < 3, e.g.: If max_write_buffer_number < 3, writing will be slowed down to options.delayed_write_rate if we are writing to the last write buffer allowed.)
• Fix max_sub_compactions describe
• rocksdb.block_size defaults are 16384 or 16Kb, no 4 Kb (as in comment)
• rocksdb.cache_index_and_filter_blocks defaults are yes instead of no in comment

[PragmaTwice]

Maybe you also need to update the default value in https://github.com/apache/kvrocks/blob/unstable/src/config/config.cc .

[aleksraiden]

Maybe you also need to update the default value in https://github.com/apache/kvrocks/blob/unstable/src/config/config.cc .

You are right, done

[caipengbo]

LGTM

[torwig]

Wanted to cancel my comments but failed :)

[mapleFU]

Great work!

Seems it doesn't change the default value because the conf file will overwrite the code default?

[aleksraiden]

Thanks, @torwig for helps with config.cc