Fix inaccurate PYTHONUNBUFFERED comment

`PYTHONUNBUFFERED` affects stdout and stderr, not stdin:
https://docs.python.org/3/using/cmdline.html#envvar-PYTHONUNBUFFERED

And updates the comment to actually explain the why not the what.

GUS-W-23030029.

Author: edmorley

bin/compile

@@ -75,7 +75,7 @@ python_home='/app/.heroku/python'
 # using `/app` paths which will still work at run-time after relocation. Amongst other things, this
 # ensures that the shebang lines in the entrypoint scripts of installed packages are correct.
 export PATH="/app/.heroku/python/bin:${PATH}"
-# Tell Python to not buffer its stdin/stdout.
+# Disable Python's output buffering to ensure logs aren't dropped if an app crashes.
 export PYTHONUNBUFFERED=1
 # Ensure Python uses a Unicode locale, to prevent the issues described in:
 # https://github.com/docker-library/python/pull/570