doc: git-checkout: reorganize examples

doc: git-checkout: reorganize examples

The examples are an ordered list, however, they are complex enough that
a callout is inside example 1, and that confuses the parsers as the list
continuation (`+`) is unclear (are we continuing the previous list item,
or the previous callout?).

We could use an open block as the asciidoctor documentation suggests,
but that has a tiny formatting issue (a newline is missing).

To simplify things for everyone (the reader, the writer, and the parser)
let's use subsections.

After this change, the HTML documentation generated with asciidoc has
the right indentation.

Cc: Jeff King <peff@peff.net>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>