Fixes #3379.

This clarifies the asynchronous file object notes for trio.wrap_file():

• aclose() on the async wrapper closes the wrapped synchronous file object.
• the async wrapper does not expose a synchronous close() shortcut.
• Trio does not guarantee that garbage collection of the async wrapper closes the wrapped file.
• continuing to use the original synchronous file object while also using the async wrapper has the same concurrency caveat as using the async wrapper from multiple tasks.

I also added a 3379.doc.rst newsfragment.

Validation:

• PATH=/Volumes/STOREJET/2/Dependencies/trio/docs-venv-313/bin:$PATH /Volumes/STOREJET/2/Dependencies/trio/docs-venv-313/bin/sphinx-build -b html -D 'exclude_patterns=._*' source build/html

Notes:

• The docs environment was installed from docs-requirements.txt into an external SSD venv using Python 3.13.
• A Python 3.14 docs environment failed while building the pinned immutables==0.21 wheel, before Sphinx ran. Python 3.13 installed the pinned docs requirements cleanly.
• The exclude_patterns=._* override is only for this local external drive, which creates AppleDouble ._* metadata files that Sphinx otherwise treats as source files.