[gdb/doc] Fix documentation of handle SIGKILL

[gdb/doc] Fix documentation of handle SIGKILL

Here ( https://sourceware.org/gdb/current/onlinedocs/gdb.html/Signals.html ) I
read:
...
GDB has the ability to detect any occurrence of a signal in your program.  You
can tell GDB in advance what to do for each kind of signal.
...

However, here ( https://man7.org/linux/man-pages/man2/ptrace.2.html ) I read:
...
While being traced, the tracee will stop each time a signal is
delivered, even if the signal is being ignored.  (An exception is
SIGKILL, which has its usual effect.)
...

So, it seems to be that for SIGKILL we can't tell GDB in advance what to do.

Fix the documentation to reflect this.

Approved-By: Eli Zaretskii <eliz@gnu.org>
PR gdb/32714
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32714