devel-docs: we must still restart a plug-in to debug with SIGCONT.

Commit 59f2ba44c7 changed the devel docs to tell that on Linux, just
running the gdb command `continue` is enough to restart the plug-in
process. Maybe it is true on some specific setups, so I leave this part
of the note, but for sure it never worked for me. I may run `continue`
as many times as I want, the process stays stopped until I explicitly
send a SIGCONT signal (our code raises itself a SIGSTOP which requires a
SIGCONT for continuation of the process).

Maybe in some configurations, gdb actually sends a SIGCONT when
`continue` is run but not in others? No idea.

Anyway I add back the part about sending a SIGCONT too, then people can
test and choose which procedure works for them.

devel-docs/debug-plug-ins.txt

@@ -69,11 +69,14 @@ To use a debugger on a C-language plug-in:
      (e.g. "gdb> attach <pid>")
      Expect the debugger to say where the plug-in is suspended.
 
-
   5. In the debugger, set breakpoints (or examine memory, or step, etc.)
 
   6. Windows: resume the plug-in process with "gimp-debug-resume.exe <pid>"
-     (On Linux, the gdb continue command resumes the attached process.)
+
+     Linux and Unix-like: the gdb `continue` command might resume the
+     attached process. Possibly you will have to send a SIGCONT signal:
+
+         kill -SIGCONT <pid>
 
   7. In the debugger, enter "continue". Expect the plug-in to resume under
      control of the debugger and pause at breakpoints.