docs: Add missing ext troubleshooting to Debian docs

Fixes #879

docs/installation/debian.rst

@@ -91,3 +91,44 @@ about any other requirements needed for the extension to work properly.
 
 For a full list of available Mopidy extensions, including those not
 installable from apt.mopidy.com, see :ref:`ext`.
+
+
+Missing extensions
+==================
+
+If you've installed a Mopidy extension with pip, restarted Mopidy, and Mopidy
+doesn't find the extension, there's probably a simple explanation and solution.
+
+Mopidy installed with APT can detect and use Mopidy extensions installed with
+either APT and pip. APT installs Mopidy as :file:`/usr/bin/mopidy`.
+
+Mopidy installed with pip can only detect Mopidy extensions installed from pip.
+pip usually installs Mopidy as :file:`/usr/local/bin/mopidy`.
+
+If you have Mopidy installed from both APT and pip, then the pip-installed
+Mopidy will probably shadow the APT-installed Mopidy because
+:file:`/usr/local/bin` usually has precedence over :file:`/usr/bin` in the
+``PATH`` environment variable. To check if this is the case on your system, you
+can use ``which`` to see what installation of Mopidy you use when you run
+``mopidy`` in your shell::
+
+    $ which mopidy
+    /usr/local/bin/mopidy
+
+If this is the case on your system, the recommended solution is to check that
+you have Mopidy installed from APT too::
+
+    $ /usr/bin/mopidy --version
+    Mopidy 0.19.5
+
+And then uninstall the pip-installed Mopidy::
+
+    sudo pip uninstall mopidy
+
+Depending on what shell you use, the shell may still try to use
+:file:`/usr/local/bin/mopidy` even if it no longer exists. Check again with
+``which mopidy`` what your shell believes is the right ``mopidy`` executable to
+run. If the shell is still confused, you may need to restart it, or in the case
+of zsh, run ``rehash`` to update the shell.
+
+For more details on why this works this way, see :ref:`debian`.