docs: clarify guarded namespaces; remove invalid namespace guards

[MSanjanaa]

Error. Please correct it. #4328
What:

• Fix incorrect documentation example that placed when(default()) guards on namespace selectors.
• Move the default() guard usage onto mixin parameters/definitions and add correct examples showing namespaced mixin usage.
• File changed: docs/features/mixins.md (branch: fix/docs-guarded-namespaces).

Why:

• The default() guard applies to mixins/parameters only, not to namespaces. The previous example was misleading and technically incorrect. This change clarifies expected behavior and provides correct examples so readers won't be confused.

Additional notes:

• This is a documentation-only change. No behavior or tests modified.
• Reviewer: please confirm phrasing and example formatting for the docs site.