docs: expand run_process Ctrl+C note

[nightcityblade]

Fixes #3300

Expands the run_process docs to mention foreground/background process-group behavior on Unix and points readers to start_new_session=True and process_group=0 when they need isolation from terminal job control.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (3dd35d7) to head (e513169).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3425   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19424        19418    -6     
  Branches         1318         1318           
===============================================
- Hits            19424        19418    -6     

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

... and 9 files with indirect coverage changes

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[A5rocks]

Sorry, I don't see how this would have helped the initial issue. The only route to get the error in the issue (that I can think of) is:

• spawn a subprocess (which is in a different process group)
• process uses tcsetpgrp on that new process group

Or alternatively, a subprocess does a similar dance. This documentation... doesn't really help. Honestly I'm not sure what would help with the issue.

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!