docs: document global variables in MDX scope

[Josh-Cena]

Motivation

Complete #5200 because that can't be merged without CLA and doesn't look like the author is ever responding.

Have you read the Contributing Guidelines on pull requests?

Yes

[netlify]

✔️ [V2]

🔨 Explore the source changes: 0b3c139

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/6187ec7c09e8a70008190705

😎 Browse the preview: https://deploy-preview-5900--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	76
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5900--docusaurus-2.netlify.app/

[github-actions]

Size Change: -3 B (0%)

Total Size: 881 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	38.2 kB	0 B
website/build/assets/css/styles.********.css	94 kB	0 B
website/build/assets/js/main.********.js	474 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	66.1 kB	0 B
website/build/blog/index.html	36.8 kB	0 B
website/build/docs/index.html	44 kB	0 B
website/build/docs/installation/index.html	51.5 kB	-3 B (0%)
website/build/index.html	29.5 kB	0 B
website/build/tests/docs/index.html	25.2 kB	0 B
website/build/tests/docs/standalone/index.html	21.7 kB	0 B

_{compressed-size-action}

[Josh-Cena]

I didn't document the __filename variable because when we migrate to ESM it may be lost (didn't investigate), and when I tested it points to index.js with no other information, and I think because this is ultimately client-side it's not going to be hugely useful.

assets and metadata are not documented either because they may be changed in the future. The other variables are more useful for the client-side user.

[slorber]

Thanks, makes sense

Not even sure we should document contentTitle though, any use-case in mind to want to use it?

[Josh-Cena]

Not even sure we should document contentTitle though, any use-case in mind to want to use it?

No, but it's just such a simple API and I don't expect it to change in the future😄 And it's less than useless compared to __filename