A few low-hanging fruit docs fixes

[negz]

• Fixes Realtime compositions are beta in 1.20, read as alpha in docs #954
• Fixes Composition function example shows modifications to composite metadata which are not supported #932
• Fixes Documentation says that Composition Functions run on deletion when they do not #931
• Fixes Document the composition response cache #926

[netlify]

✅ Deploy Preview for crossplane ready!

Name	Link
🔨 Latest commit	a21cd79
🔍 Latest deploy log	https://app.netlify.com/projects/crossplane/deploys/689cedae7cc7fe0008d17b08
😎 Deploy Preview	https://deploy-preview-971--crossplane.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 58 (🔴 down 23 from production) Accessibility: 90 (🔴 down 2 from production) Best Practices: 83 (no change from production) SEO: 100 (no change from production) PWA: 70 (no change from production) View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[jeanduplessis]

LGTM 👍