Direct user to rebuild fc-cache in font FAQ

The configuration file that's suggested to force a font to
show up as monospace is very helpful, but it appears you may
need to force fontconfig to rebuild the cache in order for the
change to take effect.

Author: ricci

docs/faq.rst

@@ -189,6 +189,11 @@ following :file:`~/.config/fontconfig/fonts.conf`::
     </match>
     </fontconfig>
 
+After creating (or modifying) this file, you may need to run the following
+command to rebuild your fontconfig cache::
+
+    fc-cache -r
+
 Then, the font will be available in ``kitty list-fonts``.