doc: fix typos

This fixes lintian I:spelling-error-in-manpage.
Note that "software" is uncountable.
deb: add limitation about architecture

This information is based on buildd on debian.org.
https://buildd.debian.org/status/package.php?p=groonga&suite=sid

To support all platforms on debian is not currently realistic,
so it is reasonable to restrict architecture explicitly.