ptrace.2: Add details about usage of PTRACE_GET_SYSCALL_INFO

ptrace.2: Add details about usage of PTRACE_GET_SYSCALL_INFO

Document the role of PTRACE_O_TRACESYSGOOD option in connection with
PTRACE_GET_SYSCALL_INFO.

Came upon this after writing a test program using
PTRACE_GET_SYSCALL_INFO.  After failing to find what's wrong I posted a
StackOverflow question which you can find right here:
<https://stackoverflow.com/questions/72410182/ptrace-get-syscall-info-always-returns-info-op-as-ptrace-syscall-info-none>

Nate Eldredge found out what happens by looking into the kernel's source
code, here is a link to the relevant part
<https://github.com/torvalds/linux/blob/8291eaafed36f575f23951f3ce18407f480e9ecf/kernel/ptrace.c#L1018>

In the code it can be seen that in case of system call entry or exit
stops, the union is filled if and only if the signal matches
`SIGTRAP | 0x80`, a signal which is only sent if the
PTRACE_O_TRACESYSGOOD option is set.  You can read about that in the
PTRACE_O_TRACESYSGOOD section of ptrace(2)'s manual.

Complements: fc91449cb "ptrace.2: Document PTRACE_GET_SYSCALL_INFO"
Cowritten-by: Dmitry V. Levin <ldv@strace.io>
Signed-off-by: Dmitry V. Levin <ldv@strace.io>
Signed-off-by: Fotios Valasiadis <fvalasiad@gmail.com>
Acked-by: Nate Eldredge <nate@thatsmathematics.com>
Cc: Elvira Khabirova <lineprinter0@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>