src/doc/en/installation/conda.rst: Add note on editable_mode=compat

Author: Matthias Koeppe

src/doc/en/installation/conda.rst

@@ -171,3 +171,11 @@ To build the documentation, use::
 
   $ pip install --no-build-isolation -v -v --editable ./pkgs/sage-docbuild
   $ sage --docbuild all html
+
+.. NOTE::
+
+   The switch ``--config-settings editable_mode=compat`` restores the
+   `legacy setuptools implementation of editable installations
+   <https://setuptools.pypa.io/en/latest/userguide/development_mode.html>`_.
+   Adventurous developers may omit this switch to try the modern,
+   PEP-660 implementation of editable installations, see :issue:`34209`.