bisect: update usage and docs to match each other

Update the usage string of `git bisect` and documentation to match each
other. While at it, also:

1. Move the synopsis of `git bisect` subcommands to the synopsis
   section, so that the test `t0450-txt-doc-vs-help.sh` can pass.

2. Document the `git bisect next` subcommand, which exists in the code
   but is missing from the documentation.

See also: [1].

[1]: https://lore.kernel.org/git/3DA38465-7636-4EEF-B074-53E4628F5355@gmail.com/

Suggested-by: Ben Knoble <ben.knoble@gmail.com>
Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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