Update guide

docs/development/developer-guide.rst

@@ -136,17 +136,30 @@ To refresh the dependencies, run the following command::
 Type annotations
 ----------------
 
-Setuptools makes use of inferred return annotations to reduce verbosity,
-especially for complex return types. Explicit return types can be added for
-functions whose inferred return type contains ``Any``, or that are not checked by
-mypy (ie.: contains no parameter *and* no return type, see
+Most standards and best practices are enforced by
+[Ruff](https://docs.astral.sh/ruff/rules/)'s ``ANN2``, ``FA``, ``PYI``, ``UP``
+and ``YTT`` rules.
+
+Explicit return types have to be added for typed public functions whose
+parameters are *all* annotated. This is enforced by `ANN2`, but it's worth noting
+that this is due to mypy inferring `Any` even for simple return types. Mypy also
+doesn't count functions with missing parameter annotations as "typed". (see
 [python/mypy#4409](https://github.com/python/mypy/issues/4409),
 [python/mypy#10149](https://github.com/python/mypy/issues/10149) and
-[python/mypy#6646](https://github.com/python/mypy/issues/6646) ).
+[python/mypy#6646](https://github.com/python/mypy/issues/6646)).  
+Otherwise, return annotations can be omitted to reduce verbosity,
+especially for complex return types.
 
 Instead of typing an explicit return type annotation as
 ``Generator[..., None, None]``, we'll prefer using an ``Iterator`` as it is more
 concise and conceptually easier to deal with. Returning a ``Generator`` with no
 ``yield`` type or ``send`` type can sometimes be considered as exposing
 implementation details. See
 [Y058](https://github.com/PyCQA/flake8-pyi/blob/main/ERRORCODES.md#Y058).
+
+Avoid importing private type-checking-only symbols. These are often
+[typeshed](https://github.com/python/typeshed) internal details and are not
+guaranteed to be stable.  
+Importing from ``_typeshed`` or ``typing_extensions`` is fine, but if you find
+yourself importing the same symbol in ``TYPE_CHECKING`` blocks a lot, consider
+implementing an alias directly in ``setuptools``.