feat(docs): describe when the Python interpreter must be bundled

docs/reference/plugins/python_plugin.rst

@@ -1,2 +1,23 @@
 
 .. include:: /common/craft-parts/python_plugin.rst
+   :end-before: .. _python-details-begin:
+
+Dependencies
+------------
+
+Whether the Python interpreter needs to be included in the snap depends on its
+``confinement``. Specifically:
+
+- Projects with ``strict`` or ``devmode`` confinement can safely use the base
+  snap's interpreter, so they typically do **not** need to include Python.
+- Projects with ``classic`` confinement **cannot** use the base snap's
+  interpreter, and thus must always bundle it (typically via ``stage-packages``).
+- In both cases, a specific/custom Python installation can always be included
+  in the snap. This can be useful, for example, when using a different Python
+  version or building an interpreter with custom flags.
+
+Snapcraft will prefer an included interpreter over the base's, even for projects
+with ``strict`` and ``devmode`` confinement.
+
+.. include:: /common/craft-parts/python_plugin.rst
+   :start-after: .. _python-details-end: