doc/git-bisect: clarify `git bisect run` syntax

doc/git-bisect: clarify `git bisect run` syntax

The description of the `git bisect run` command syntax at the beginning
of the manpage is `git bisect run <cmd>...`, which isn't quite clear
about what `<cmd>` is or what the `...` mean; one could think that it is
the whole (quoted) command line with all arguments in a single string,
or that it supports multiple commands, or that it doesn't accept
commands with arguments at all.

Change to `git bisect run <cmd> [<arg>...]` to clarify the syntax,
in both the manpage and the `git bisect -h` command output.

Additionally, change `--term-{new,bad}` et al to `--term-(new|bad)`
for consistency with the synopsis syntax conventions.

Signed-off-by: Javier Mora <cousteaulecommandant@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>