docs(index): fix stale default values in HnswBuildParams doc comments

[wombatu-kun]

The three builder methods on HnswBuildParams document default values that no longer match impl Default.

Method	Doc said	Default gives
max_level	8	7
num_edges	30	20
ef_construction	100	150

The format spec table in docs/src/format/index/vector/index.md already lists the correct values, so these builder doc comments are the only place left stating the old ones.

Also fixes the max_level field doc, which currently reads "max level ofm".

Docs only, no behavior change.

One question while I am here: lance.util.HNSW.build in the Python bindings still defaults ef_construction to 100, the value the Rust default carried before it moved to 150. That is a real default rather than a doc, so I left it untouched. Should it be lined up with 150? I can fold that into this PR or send it as a separate follow-up, whichever you prefer.

[lance-gatekeeper]

Gate recommendation: approve. The stale Rust builder documentation now matches HnswBuildParams::default() and the format specification, with no runtime or compatibility impact.