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 <>
Reviewed-by: Philippe Mathieu-Daudé <>
Reviewed-by: Richard Henderson <>
Reviewed-by: Alex Bennée <>
2 files changed