docs: Improve our gdbstub documentation

The documentation of our -s and -gdb options is quite old; in
particular it still claims that it will cause QEMU to stop and wait
for the gdb connection, when this has not been true for some time:
you also need to pass -S if you want to make QEMU not launch the
guest on startup.

Improve the documentation to mention this requirement in the
executable's --help output, the documentation of the -gdb option in
the manual, and in the "GDB usage" chapter.

Includes some minor tweaks to these paragraphs of documentation
since I was editing them anyway (such as dropping the description
of our gdb support as "primitive").

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200403094014.9589-1-peter.maydell@linaro.org

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