docs/manual: cleanup github helper docs

Explicitly state that the github helper should not be used when there is
a release tarball.

Properly render the list by separating it from the previous paragraph.

[Peter: fix typo as pointed out by Maxime]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>