documentation for making git add only annex when configured by annex.largefiles

Code change should be trvial, but not yet implemented. This
significantly complicated the task of documenting how git-annex works.

I'm not sure how useful the annex.gitaddtoannex confguration is after
this change; seems that if a user has an annex.largefiles they will want
it applied consistently. But the last thing I want to hear is more
complaining from users about git add doing something they don't want it
to.

There's a pretty high risk users who got used to the git add behavior
and don't have annex.largefiles configured will miss the NEWS and
complain bitterly about their suddenly bloated repositories. Oh well.

Removed outdated comments about the old behavior to avoid confusion.
I don't know if I've found all the places that griping spread to.