diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index 23e210d..8deba39 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -649,6 +649,13 @@
 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*