Issue 15157: Adjust building blocks docs

[jmh530]

No description provided.

[quickfur]

LGTM

[dlang-bot]

Thanks for your pull request, @jmh530! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.

Some tips to help speed things up:

• smaller, focused PRs are easier to review than big ones

• try not to mix up refactoring or style changes with bug fixes or feature enhancements

• provide helpful commit messages explaining the rationale behind each change

Bear in mind that large or tricky changes may require multiple rounds of review and revision.

Please see CONTRIBUTING.md for more information.

Bugzilla references

Auto-close	Bugzilla	Description
✓	15157	std.experimental.allocator.building_blocks docs

[wilzbach]

(rebased to upstream/master to fix the CI failures)

[wilzbach]

I just noticed the following:

As explained here:

A ✗ means that the issue is only mentioned (i.e. the commit message doesn't contain "Fix") and the Bugzilla issue won't be auto-closed nor listed in the changelog as fixed:

-> I prefixed the commit title with "Fix "