split dev_extras into dev_extras+docs_extras

readthedocs.org.requirements.txt

@@ -1,10 +1,10 @@
 # readthedocs.org gives no way to change the install command to "pip
-# install -e .[dev]" (that would in turn install documentation
+# install -e .[docs]" (that would in turn install documentation
 # dependencies), but it allows to specify a requirements.txt file at
 # https://readthedocs.org/dashboard/letsencrypt/advanced/ (c.f. #259)
 
 # Although ReadTheDocs certainly doesn't need to install the project
-# in --editable mode (-e), just "pip install .[dev]" does not work as
-# expected and "pip install -e .[dev]" must be used instead
+# in --editable mode (-e), just "pip install .[docs]" does not work as
+# expected and "pip install -e .[docs]" must be used instead
 
--e .[dev]
+-e .[docs]