docs: clarify wrap_file ownership and lifecycle

[nightcityblade]

Fixes #3379

Document what happens to the wrapped sync file when the async wrapper is closed, whether garbage collection closes it automatically, and why the original sync file should not be used concurrently while wrapped.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (3dd35d7) to head (e411fac).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3423   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19424        19436   +12     
  Branches         1318         1318           
===============================================
+ Hits            19424        19436   +12     

Files with missing lines	Coverage Δ
src/trio/_file_io.py	100.00000% <ø> (ø)
src/trio/_tests/test_file_io.py	100.00000% <100.00000%> (ø)

... and 9 files with indirect coverage changes

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[A5rocks]

1. please make CI pass
2. if we don't have tests for this behavior, writing this out in the docs means we promise it... so please add tests!

[A5rocks]

This doesn't make sense, that's literally the file handle in question?

[A5rocks]

I think maybe this should be indented into the returns section? Not sure about how that would look formatting-wise.

[nightcityblade]

Thanks, that makes sense. Rather than land wording that over-promises behavior without coverage, I opened a separate docs-only clarification for the related run_process job-control pitfall and I will leave this one for a proper rework with passing CI and tests.

[A5rocks]

I'm not sure how your comment is related to this PR. What does wrap_file have to do with run_process?

Anyways, it seems you have been opening too many PRs recently... 68 in the last 2 weeks! Unfortunately I think the open issues on Trio need a bunch of thought to how to solve them, so I'm going to close this. Feel free to reopen it whenever you've slowed down how many PRs you make!