Docs fixes from c8594804 audit: Bet365 tier (Pro→Hobby), SSE channel CSV collapse, is_main_line vs market=main, _bN grouping recipe

[Mlaz-code]

Doc gaps surfaced by the Crisp c8594804 audit (eng verdicts in Mlaz-code/sharp-api-go#885). Four fixes, all verified against code/prod on 2026-06-11:

• Bet365 tier contradiction — content/en/concepts/live-vs-prematch.mdx tier table says Bet365 requires Pro; code truth is RequiresTier: "hobby" (sharp-api-go sportsbooks.go:33-34) and api-reference/sportsbooks.mdx correctly lists it under Hobby. Change live-vs-prematch to Hobby.
• SSE channel CSV semantics — api-reference/stream.mdx documents channel as a single value. Since sharp-api-go #610 (2026-05-22) the endpoint accepts channel/channels as a comma-separated list, but SSE supports one subscription per stream, so >1 valid channels collapse to all. Document this (and contrast with WS, which supports true selective multi-channel via channels=).
• is_main_line vs market=main on /odds/best — is_main_line is per-(event, market, selection) cohort: it marks the main line of that market's own ladder, so 8th_inning_total_runs / props rows legitimately carry is_main_line=true. Customers wanting "main game lines only" need ?market=main. Spell this out on the /odds/best page (this was audit item D1; the semantics are intended but undocumented).
• _bN grouping recipe — concepts/event-matching.mdx documents the bucket suffix (SHA-3432 C3, 2026-05-21) but gives no concrete cross-bucket grouping recipe. Add: strip trailing _b{N} (and _g{N} doubleheader suffix) and group by {league}_{home}_{away}_{date}, tolerating ±1 day for cross-midnight starts (mirror of strip_doubleheader_suffix in api-adapters utils/canonical_ids.py:406).