docs: clarify quota estimates for usage limits

[binichallein]

Related Issue

Resolve #2278

Description

• Clarify that membership request counts are rough estimates derived from token consumption rather than fixed per-request allowances.
• Point users to /usage as the source of truth for remaining quota.
• Mention that long context, large files, images, tool output, subagents, and repeated edits can consume quota faster than short Q&A.

Verification

• git diff --check
• npx -y node@20 scripts/sync-changelog.mjs && npx -y node@20 node_modules/vitepress/bin/vitepress.js build

Note: npm run build with the system Node v18.19.1 fails before this change because vitepress-plugin-llms requires Node 20 APIs (Array.prototype.toReversed). The docs CI uses Node 20, so the verification above runs the same Node major version.

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have run docs build verification for this docs-only change.
• I have added tests that prove my fix is effective or that my feature works. (N/A: documentation-only change.)
• I have run make gen-changelog to update the changelog. (N/A: no changelog change.)
• I have run make gen-docs to update the user documentation. (N/A: this PR directly edits the user documentation.)

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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