Some documentation issues

[bluebad]

I've encountered several documentation issues.

In tantivy.py, the comment documentation for the Snippet class is incorrect.

Additionally, after implementing a piece of code for adding indexes based on the example, I faced a "ValueError: An error occurred in a thread: 'An index writer was killed.. A worker thread encountered an error (io::Error most likely) or panicked.'" after calling index writer.commit() tens of thousands of times. However, the issue was resolved after following the example in the tests directory, where writer.wait_merging_threads() is called after multiple commits. I am not sure if this is an issue on my end, but if it's not, then the documentation should be updated to clarify this process. Thank you for your work!

[cjrh]

The documentation is very rough and we know there is a lot of work to be done. We're all volunteers though so fixes happen as time allows.

comment documentation for the Snippet class is incorrect.

Could you specify in what way it's incorrect? This will help anyone else who comes along and has a few minutes to spare to submit a quick fix for a low-hanging-fruit issue.

after calling index writer.commit() tens of thousands of times

You should only be calling commit after adding a batch of documents, not after each individual document.

[bluebad]

Thank you for your hard work. I just hope to make a contribution to this great project. My comments are not meant to offend or criticize.
Below is the docstring for the Snippet:

class Snippet(object):
    """
    Tantivy schema.
    
    The schema is very strict. To build the schema the `SchemaBuilder` class is
    provided.
    """

It seems to have mistakenly used the documentation for Schema.

[bluebad]

You should only be calling commit after adding a batch of documents, not after each individual document.

I did call add_document multiple times before committing, approximately once every 1000 calls, but I encountered the error mentioned above. It was not until I used wait_merging_threads that I was able to successfully complete the entire indexing process.