Add TIP explaining IntelliSense behavior inside comments and string literals

[NA-V10]

This PR adds a helpful TIP to intellisense.md explaining that IntelliSense does
not appear inside comments or regular string literals unless the language extension
explicitly supports it.

✔ Why this is useful

Many users expect IntelliSense everywhere and may think it is broken when typing
inside comments or strings. This clarification improves accuracy and helps avoid
confusion.

Checklist

• Documentation only (no functional changes)
• Follows VS Code Docs callout format
• Change added in correct section
• Branch created from fork, targeting main

[NA-V10]

@microsoft-github-policy-service agree