Make the noxfile support passing a less temporary directory for build…

…ing docs.

noxfile.py

@@ -155,15 +155,16 @@ def docs(session, builder):
         argv = ["-n", "-T", "-W"]
         if builder != "spelling":
             argv += ["-q"]
+        posargs = session.posargs or [tmpdir / builder]
         session.run(
             "python",
             "-m",
             "sphinx",
             "-b",
             builder,
             DOCS,
-            tmpdir / builder,
             *argv,
+            *posargs,
         )