doc: bring a few examples up to date.

doc: bring a few examples up to date.

* doc/automake.texi (Built Sources Example): use @kbd for typed
commands; show current output.
(Checking the Distribution): refer to Third-Party Makefiles
for the do-nothing dvi target example.
Other small wording tweaks throughout.

* HACKING: consistently omit $ prompt.
Say a bit more about versions of Autoconf.
Describe actual current status of branches (only master in use).
Give some topic examples.
"rerun" not "re-run".
Other small wording tweaks throughout.