patch 8.0.1713: terminal debugger doesn't handle arguments

Problem:    Terminal debugger doesn't handle arguments.
Solution:   Use <f-args> and pass all the arguments to gdb, e.g. the core file
            or process number. (suggested by Christian Brabandt) Disallow
            starting the debugger twice.

runtime/doc/terminal.txt

@@ -649,6 +649,13 @@ the same as any command running in a terminal window.
 When the debugger ends, typically by typing "quit" in the gdb window, the two
 opened windows are closed.
 
+Only one debugger can be active at a time.
+
+To attach gdb to an already running executable, or use a core file, pass extra
+arguments.  E.g.: >
+	:Termdebug vim core
+	:Termdebug vim 98343
+
 
 Example session ~
 							*termdebug-example*