Fix Issue 7446 - [TDPL] Trivial asynchronous file copy example crashes

[ghost]

The original issue (docs telling, that the runtime catches the exception) seems not to exist anymore, but anyway, the functions should document, that they might throw. i also cleaned up the docs of the three functions a little bit.

[dlang-bot]

Thanks for your pull request and interest in making D better, @berni44! 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

Auto-close	Bugzilla	Severity	Description
✓	7446	normal	[TDPL] Trivial asynchronous file copy example crashes with OwnerTerminated

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7333"

[atilaneves]

The "docs" in issue 7446 is TDPL, meaning that someone who read the book would get behaviour different from described there.

[ghost]

The "docs" in issue 7446 is TDPL, meaning that someone who read the book would get behaviour different from described there.

Ah, I thought TDPL was just an old way to reference the D docs.

The changes i suggest (including closing the issue) make sense anyway, don't they?

[ghost]

ping?