Docs: Always rebuild all docs

Previously, only calling "make clean && make html" would display all
errors and warnings in the docs. This was too slow to do by default, and
hence many errors would go unnoticed.
Adding the -a flag to Sphinx forces it to look at all the files again,
but is considerably quicker than deleting the build directory. As such,
I think it's good default behaviour as it isn't too slow even on
non-SSD machines.