manual: Update signal descriptions

[DJ Delorie]

Based on auditing all the signals and source trees for Hurd and
Linux...

SIGSYS - This is not used for a bad system call (ENOSYS is used
for that).  This is used by SECCOMP and some cases where an invalid
sub-function was requested.

SIGSTKFLT - Note it used to be a coprocessor stack fault but is now
obsolete and available for general user use.

SIGLOST - Hurd only now; note that its original purpose as an NFS
lock lost signal is obsolete.

SIGPWR - Note this is for power lost *and* power restored, and is
more a user-mode signal than a kernel-generated signal.

diff --git a/manual/signal.texi b/manual/signal.texi
index 2012980efe..9d7f8667e5 100644
--- a/manual/signal.texi
+++ b/manual/signal.texi
@@ -427,11 +427,21 @@ failure to properly emulate them.
 
 @deftypevr Macro int  SIGSYS
 @standards{Unix, signal.h}
-Bad system call; that is to say, the instruction to trap to the
-operating system was executed, but the code number for the system call
-to perform was invalid.
+System call event.  This signal may be generated by a valid system
+call which requested an invalid sub-function, and also by the SECCOMP
+filter when it filters a system call.
+
+If the system call itself is invalid or unsupported by the kernel, the
+call will not raise this signal, but will return ENOSYS.
+@end deftypevr
+
+@deftypevr Macro int  SIGSTKFLT
+Coprocessor stack fault.  Obsolete, no longer generated.  This signal
+may be used by applications in much the way @code{SIGUSR1} and
+@code{SIGUSR2} are.
 @end deftypevr
 
+
 @node Termination Signals
 @subsection Termination Signals
 @cindex program termination signals
@@ -752,12 +762,11 @@ that isn't connected.  @xref{Sending Data}.
 @deftypevr Macro int SIGLOST
 @standards{GNU, signal.h}
 @cindex lost resource signal
-Resource lost.  This signal is generated when you have an advisory lock
-on an NFS file, and the NFS server reboots and forgets about your lock.
-
-On @gnuhurdsystems{}, @code{SIGLOST} is generated when any server program
-dies unexpectedly.  It is usually fine to ignore the signal; whatever
-call was made to the server that died just returns an error.
+Resource lost.  On @gnuhurdsystems{}, @code{SIGLOST} is generated when
+any server program dies unexpectedly.  It is usually fine to ignore
+the signal; whatever call was made to the server that died just
+returns an error.  This signal's original purpose of signalling a lost
+NFS lock is obsolete.
 @end deftypevr
 
 @deftypevr Macro int SIGXCPU
@@ -817,6 +826,17 @@ to print some status information about the system and what the process
 is doing.  Otherwise the default is to do nothing.
 @end deftypevr
 
+@deftypevr Macro int SIGPWR
+@cindex power event signal
+Power lost or restored.  On s390x Linux systems, this signal is
+generated when a machine check warning is issued, and is sent to the
+process designated to receive ctrl-alt-del notificiations.  Otherwise,
+it is up to userspace applications to generate this signal and manage
+notifications as to the type of power event that happened.
+
+The default action is to terminate the process.
+@end deftypevr
+
 @node Signal Messages
 @subsection Signal Messages
 @cindex signal messages