docs: 'dist-shar' and 'dist-tarZ' are obsolescent today

docs: 'dist-shar' and 'dist-tarZ' are obsolescent today

Both the options and the formats; and they might be deprecated
and removed in future automake versions (see discussion on
automake bug#13324).

In any case, it's better if the documentation starts advising
against their use right now.

* doc/automake.texi (The Types of Distributions): Here.
(List of Automake options): And here.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>