doc: add documentation about os.tmpdir() overrides

This documents the TMPDIR, TEMP and TMP overrides on different
platforms and that some operating systems set these by default.

PR-URL: #54613
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: joyeecheung

doc/api/os.md

@@ -416,6 +416,19 @@ changes:
 Returns the operating system's default directory for temporary files as a
 string.
 
+On Windows, the result can be overridden by `TEMP` and `TMP` environment variables, and
+`TEMP` takes precedence over `TMP`. If neither is set, it defaults to `%SystemRoot%\temp`
+or `%windir%\temp`.
+
+On non-Windows platforms, `TMPDIR`, `TMP` and `TEMP` environment variables will be checked
+to override the result of this method, in the described order. If none of them is set, it
+defaults to `/tmp`.
+
+Some operating system distributions would either configure `TMPDIR` (non-Windows) or
+`TEMP` and `TMP` (Windows) by default without additional configurations by the system
+administrators. The result of `os.tmpdir()` typically reflects the system preference
+unless it's explicitly overridden by the users.
+
 ## `os.totalmem()`
 
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