git-pack-objects(1): --honor-pack-keep

Hi All,

the option `--honor-pack-keep` is not mentioned in the `SYNOPSIS` of `git-pack-objects(1)` (`Documentation/git-pack-objects.txt`), only below in the `OPTIONS`.

The same story with `--include-tag`, `--window-memory`, `--max-pack-size`, `--no-reuse-object`, `--compression`, `--thin`, `--threads`, and `--index-version`.

Is it a mistake, or different options should be treated differently, like, often used vs. seldom used? But as far as I can see, `--honor-pack-keep` is used pretty often by `git gc`.

PS The `--non-empty` block is indented with whitespaces, should it be corrected?