docs: document HIP-3 dex parameter on Info methods

[claygeo]

Summary

Adds the missing dex parameter documentation to the Args docstrings of the five Info methods that already accept and forward a dex argument for HIP-3 builder-deployed perpetuals: user_state, open_orders, frontend_open_orders, all_mids, and meta.

Fixes #286.

Why

HIP-3 bot developers don't realize these methods support per-DEX selection, because the dex parameter is only discoverable by reading the source. Documenting it in the Args section makes HIP-3 support visible.

Changes

• hyperliquid/info.py: add dex to the Args docstrings of the 5 methods that already accept it. No code or signature changes.

Scope note (this PR originally also targeted #287)

This PR initially also added a dex parameter to user_fills and user_fills_by_time. I removed that half after testing the live mainnet /info API: userFills/userFillsByTime returned an identical result set whether dex was omitted, set to a valid HIP-3 dex (xyz), or set to an invalid value (and the invalid value was not rejected) — dex appears to have no effect on those two endpoints. By contrast the sibling endpoints (clearinghouseState, openOrders, metaAndAssetCtxs) validate dex (HTTP 500 on an invalid value) and return dex-specific data. Adding a forwarded-but-ignored dex to the two fill methods would document filtering the SDK can't actually provide, so it's been dropped. Repro on #287.

Backwards compatibility

Docstring-only change. No behavior changes.

[claygeo]

Rebased onto the latest master and refreshed this PR.

One scope change worth flagging: I pared it down to the docs-only half, documenting dex on the five Info methods that already support it (user_state, open_orders, frontend_open_orders, all_mids, meta), which closes #286.

I dropped the original user_fills / user_fills_by_time change after testing the live API: the /info endpoint silently ignores a dex field on userFills/userFillsByTime (a valid HIP-3 dex returns the same default-dex fills, and an invalid dex isn't rejected), while clearinghouseState/openOrders validate it. Adding the parameter there would have implied per-DEX filtering the backend doesn't actually do. Full repro on #287.

Tests and mypy pass locally. Would appreciate a re-review when you get a chance. Thanks!