docs(api-reference): clarify market= vs market_type= and league slug vs market_segment

[sharp-resolver]

Fixes #2374

Sentinel logged ~380 customer 400 errors on 2026-07-05 across three patterns:

• market= passed with a market_type=-style value (92 hits, 5 customers)
• offset > 500 cap (169 hits — covered by the existing offset docs, no change needed)
• league= passed a market-segment slug like nfl_1st_quarter_spreads (19 hits)

PRs #279 (market_type= alias note in param row) and #280 (common-4xx-errors page covering offset/paths/streaming/unknown-slug) already landed. This PR adds the two remaining missing pieces:

Changes

common-4xx-errors.mdx

Two new sections added after the offset section:

1. market= vs market_type= confusion — explains the query param (market=) vs response field (market_type) naming difference, with a lookup table (alias vs exact type), and notes that market_type= is accepted as an alias.
2. League slug vs market-segment slug — shows the exact 400 invalid_filter body when league=nfl_1st_quarter_spreads is passed, with the fix (use market_segment= instead, fetch slugs from /api/v1/leagues).

Updated description from "four" → "six" client-side mistakes.

odds.mdx

• Sharpened the market= param row to explicitly cross-reference the market_type response field by name, so the connection is clear at a glance.
• Added two <Callout type="warning"> boxes before the Market Category Aliases section — one per confusion pattern — with direct links to /api/v1/leagues and the common-4xx-errors page.

Gate

  CHECK 1   WARN  no changed test files (verify the fix truly needs none)
  CHECK 2   PASS  no file paths claimed in body
  CHECK 3   PASS  no leaked state in diff
  CHECK 4   PASS  no workflow changes
  CHECK 5   PASS  no binary fixtures changed

GATE: PASS

Type: docs