Fix and document `output_block_log_path` of block log `truncate` method

`compress_block_log` takes output directory as argument, not target block log
file path. Previously block log name in `output_block_log_path` parameter was
ignored. So if path `/a/b/c` was requested, then block log was stored in right
directory (`/a/b`), but with bad name `/a/b/block_log`.

Now it is clearly stated, that name of output block log cannot be requested,
because parameter is named `output_directory`. Predefined name of output block
log is mentioned in documented.

Additionally missing documentation and type annotations were added.