docs(rfd): clarify usage accounting placeholders

[kocaemre]

Summary

• Removes the provisional "across session / across all turns" wording from unstable end-turn Usage fields.
• Aligns v1/v2 generated draft schema docs with the End-Turn Token Usage RFD language while leaving the per-turn vs cumulative decision open.

Verification

• cargo +nightly run -p schema-generator --features unstable
• cargo +nightly run -p schema-generator --features unstable,unstable_protocol_v2
• npm run spellcheck
• cargo +nightly check -p agent-client-protocol-schema --features unstable,unstable_protocol_v2
• cargo +nightly fmt --all -- --check
• npx prettier --check docs/rfds/end-turn-token-usage.mdx docs/protocol/v1/draft/schema.mdx docs/protocol/v2/draft/schema.mdx schema/v1/schema.unstable.json schema/v2/schema.unstable.json
• git diff --check

Closes #1860