docs: reference pages match the gateway users actually call (issue #68)

[SebConejo]

Applies the recommended options for all ten findings of #68 (batch pass, 2026-08-18), deduplicated against the already-merged PRs #62 and #65:

1. Rate limits: the section states the real fixed caps (200/min per workspace, 500/min per IP, 10 in flight — same on Cloud and self-hosted, no setting moves them), and the THROTTLE_* rows are rescoped to the dashboard's own API calls. When platform PR #2724 (env-configurable caps, still open) is merged and released, a follow-up documents the real overrides.
   2-3. 424 and the error envelope: the api.mdx and headers.mdx halves were already fixed by docs: llm-gateway and API reference drop the 424 myth (issue #53) #62; this PR fixes the glossary's last 424 and adds the provider-forwarded-429 clause to the status table.
2. Tier and reason headers: live-verified on a local instance — a Manifest-answered request reports X-Manifest-Tier: simple with model and provider manifest, values the old rows said could not occur. Rows rewritten to the values a client can receive; the tier's name appears in the dashboard, never in a header.
3. Body forwarding: "forwarded verbatim" becomes what Manifest does — converts per provider, renames max_tokens, merges harness defaults, drops OpenAI-only fields outside OpenAI/OpenRouter routes.
4. Already fixed by docs: LAN exposure goes through HOST_BIND_ADDRESS in .env (issue #56) #65 (nothing to do).
5. anthropic-version: accepted and ignored; Manifest sets its own upstream version.
6. Header beats model ID: a matching custom-tier header takes precedence over an explicit model ID — stated on the glossary, headers page, and (parked item, same wording) the routing page.
7. Client-kind split: chat/streaming clients get errors as 200-with-message, stated above the status table; the headers section heading stops promising "every request".
8. M302 softened to the connected-provider condition, matching the other two pages.

Also consumes the parked self-hosted.mdx line (drops "rate limits" from the optional-env list).

Coherence: GREEN. Closes #68.