Add FAQ entry about doctest formatting

docs/faq.md

@@ -100,6 +100,16 @@ codebase with _Black_.
 
 Yes, there is support for PyPy 3.8 and higher.
 
+## Does Black format docstrings?
+
+Yes, Black cleans up the indentation of docstrings and removes superfluous whitespace,
+but it does not format the code inside docstrings (including doctests).
+
+If you need to format doctests, consider using a dedicated tool such as
+[blacken-docs](https://github.com/adamchainz/blacken-docs) or
+[pytest-doctestplus](https://pypi.org/project/pytest-doctestplus/) with
+`pytest --doctest-modules --doctest-format=pep8`.
+
 ## Why does Black not detect syntax errors in my code?
 
 _Black_ is an autoformatter, not a Python linter or interpreter. Detecting all syntax