Remove requirements/docs.txt comment

Remove the comment about limiting `sphinx-autodoc-typehints` upper-bound now that tox-dev/sphinx-autodoc-typehints#259 is fixed.

requirements/docs.txt

@@ -1,7 +1,7 @@
 # dependencies for building docs, separate from dev.txt as this is also used for builds on readthedocs.org
 # core dependencies
 sphinx>=4.2.0, <6.1.0
-sphinx-autodoc-typehints>=1.12.0, <1.20.1  # limited due to https://github.com/tox-dev/sphinx-autodoc-typehints/issues/259 and 260 
+sphinx-autodoc-typehints>=1.12.0, <1.20.1
 sphinx-rtd-theme>=1.0.0, <2.0.0
 sphinxcontrib-apidoc>=0.3.0, <0.4.0
 sphinxcontrib-bibtex>=2.1.0, <3.0.0