Merge pull request sphinx-doc#9812 from not-my-profile/doc-theme-resp…

…onsiveness

Document responsiveness of themes

doc/usage/theming.rst

@@ -127,6 +127,10 @@ Builtin themes
 
 Sphinx comes with a selection of themes to choose from.
 
+Note that from these themes only the Alabaster and Scrolls themes are
+mobile-optimated, the other themes resort to horizontal scrolling
+if the screen is too narrow.
+
 .. cssclass:: clear
 
 These themes are: