Fix some docs and docstrings

[ajoubertza]

Minor things spotted while reading through, and doing some testing.

[jairomoldes]

Seem like this link:
http://www.tango-controls.org/resources/documentation/kernel/
should be:
http://www.tango-controls.org/documentation/kernel/

Lines 332 and 409 in howto.rst seem overindented.

[tiagocoutinho]

LGTM if you fix link according to @jairomoldes comment

[ajoubertza]

Thanks @jairomoldes and @tiagocoutinho. I've made the changes.