git-shortlog.txt: reorder usages

The first usage we give is the original one where, e.g., `git log` is piped through `git shortlog`. The description that follows reads the other way round, by first focusing on the general behavior, then ending with the behavior when reading from stdin. It is also a tiny bit odd that what is probably the most common usage and the one a reader is probably looking for is not at the top of the list. Of course, it is only a two-item list, so it is not _that_ hard to find... The next commit will add the original usage to the usage string in builtin/shortlog.c, and it feels more natural to do so below the most common usage. To avoid being inconsistent, reorder these two usages here first. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>