doc: document buildPython* stdenv overriding via <function>.override

doc/languages-frameworks/python.section.md

@@ -557,6 +557,19 @@ are used in [`buildPythonPackage`](#buildpythonpackage-function).
   with the `pipInstallHook`.
 - `unittestCheckHook` will run tests with `python -m unittest discover`. See [example usage](#using-unittestcheckhook).
 
+#### Overriding build helpers {#overriding-python-build-helpers}
+
+Like many of the build helpers provided by Nixpkgs, Python build helpers typically provide a `<function>.override` attribute.
+It works like [`<pkg>.override`](#sec-pkg-override), and can be used to override the dependencies of each build helper.
+
+This allows specifying the stdenv to be used by `buildPythonPackage` or `buildPythonApplication`. The default (`python.stdenv`) can be overridden as follows:
+
+```nix
+buildPythonPackage.override { stdenv = customStdenv; } {
+  # package attrs...
+}
+```
+
 ## User Guide {#user-guide}
 
 ### Using Python {#using-python}

doc/redirects.json

@@ -3807,6 +3807,9 @@
   "buildpythonpackage-parameters": [
     "index.html#buildpythonpackage-parameters"
   ],
+  "overriding-python-build-helpers": [
+    "index.html#overriding-python-build-helpers"
+  ],
   "overriding-python-packages": [
     "index.html#overriding-python-packages"
   ],

doc/release-notes/rl-2511.section.md

@@ -314,6 +314,10 @@
 
 - `emacs` now disables the GC mark trace buffer by default. This improves GC performance by 5%, but can make GC issues harder to debug. This is configurable with `withGcMarkTrace`.
 
+- Passing `stdenv` to `buildPythonPackage` or `buildPythonApplication` has been deprecated and will trigger an error in a future release.
+  Instead, you should _override_ the python build helper, e.g., `(buildPythonPackage.override { stdenv = customStdenv; })`.
+  See [](#overriding-python-build-helpers).
+
 - `buildPythonPackage` and `buildPythonApplication` now default to `nix-update-script` as their default `updateScript`. This should improve automated updates, since nix-update is better maintained than the in-tree update script and has more robust fetcher support.
 
 - `plasma6`: Fixed the `ksycoca` cache not being re-built when `$XDG_CACHE_HOME` is set to something that isn't `$HOME/.cache`.