diff --git a/doc/conf.py b/doc/conf.py
index c2ebc772..bfb596b9 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -71,15 +71,7 @@
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ["_static"]
 
-# http://read-the-docs.readthedocs.org/en/latest/theme.html#how-do-i-use-this-locally-and-on-read-the-docs
-# on_rtd is whether we are on readthedocs.org
-on_rtd = os.environ.get("READTHEDOCS", None) == "True"
-if not on_rtd:
-    # Import and set the theme if we're building docs locally
-    import sphinx_rtd_theme
-
-    html_theme = "sphinx_rtd_theme"
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+html_theme = "sphinx_rtd_theme"
 
 nbsphinx_kernel_name = "python3"
 nbsphinx_execute_arguments = [
diff --git a/src/iminuit/cost.py b/src/iminuit/cost.py
index b8277502..76a75436 100644
--- a/src/iminuit/cost.py
+++ b/src/iminuit/cost.py
@@ -254,7 +254,7 @@ def poisson_chi2(n: ArrayLike, mu: ArrayLike) -> float:
     n : array-like
         Observed counts.
     mu : array-like
-        Expected counts per bin. Must satisfy sum(mu) == sum(n).
+        Expected counts per bin.
 
     Returns
     -------
@@ -265,6 +265,8 @@ def poisson_chi2(n: ArrayLike, mu: ArrayLike) -> float:
     -----
     The implementation makes the result asymptotically chi2-distributed,
     which helps to maximise the numerical accuracy for Minuit.
+
+    If sum(mu) == sum(n), the result is equal to :func:`multinomial_chi2`.
     """
     n, mu = np.atleast_1d(n, mu)
     return 2 * np.sum(n * (log_or_zero(n) - log_or_zero(mu)) + mu - n)
@@ -286,7 +288,7 @@ def multinomial_chi2(n: ArrayLike, mu: ArrayLike) -> float:
     n : array-like
         Observed counts.
     mu : array-like
-        Expected counts.
+        Expected counts. Must satisfy sum(mu) == sum(n).
 
     Returns
     -------