[spec/expression] Improve placement new docs

[ntrel]

Rename subheading Placement New.
Add grammar link.
Remove 'PlacementExpression can be a dynamic array' - this is now an error, see dlang/dmd#22164 and linked PR.
Fix paragraph tag for class PlacementExpression.
Use list for restrictions.
PlacementExpression type must be mutable and not shared.

[dlang-bot]

Thanks for your pull request and interest in making D better, @ntrel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.