config: document the purpose of the `format` config section
The config section now have a top level documentation to clarify its intend and
usage. In particular, user are now explain when the option are taken in account
and how to convert repository.
There are an handful of experimental options in this section that does not match
its definition. They should be relocated to other section before getting out of
experimental. (see next changeset for one example).
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present