Document how to create FTS index in LanceDB `AsyncTable`

[prrao87]

LanceDB OSS doesn't support create_fts_index with an AsyncTable. We need to document how to use create_index when the user works with an async connection in OSS.

Here's a minimal example that works.

import asyncio
import lancedb
import polars as pl
from lancedb.index import FTS

data = pl.DataFrame(
    {
        "id": [1, 2],
        "text": ["His first language is spanish", "Her first language is english"],
    }
)

async def main(data: pl.DataFrame):
    uri = "ex_lancedb"
    db = await lancedb.connect_async(uri)
    tbl = await db.create_table("my_text", data=data, mode="overwrite")

    await tbl.create_index("text", config=FTS(language="English"))

    response = await tbl.search("spanish", query_type="fts")

    result = await response.limit(1).to_polars()
    print(result)


if __name__ == "__main__":
    asyncio.run(main(data))