doc: mention xz -T option for XZ_OPT, other wording.

* doc/automake.texi (dist-xz): wording.
(Vala Support, Python): keep @def... definitions all on one line, as
required by Texinfo; the log files gets a Warning: line.
* HACKING: mention reply-all and patch inclusion for bug lists.
Other minor wording tweaks.