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}