[DOCS] Clarify whether extra usage applies to the weekly limit

Problem

The extra usage documentation explains that extra usage lets you continue working after hitting your "included usage limit" at standard API rates. However, it only mentions the session limit (5-hour rolling window) — it never mentions the weekly limit.

This leaves a gap: does extra usage kick in when you hit the weekly limit, or only when you hit the session limit?

Why it matters

Users planning their work and budgets need to know whether hitting the weekly cap means a hard stop until reset, or whether extra usage will let them keep going (at API rates). The answer significantly affects which plan tier to choose and whether to enable extra usage.

Suggested fix

Add a sentence to the extra usage article explicitly stating which limits extra usage applies to — session only, weekly only, or both.