More formal definition of minor changes

[Andrea D'Amore]

We should add a more explicit definition of what a "minor change" is
to both the non-maintainer update policy section in the Guide and to
the NewCommittersGuide page on wiki.

I remember what a minor change is topic has been discussed but I
couldn't find it on the mailing list archive, possibly it happened on
IRC.

The most similar topic I could find is [1]

--
Andrea