Add note about 1 connection and related disconnect

ynca/terminal.py

@@ -21,10 +21,18 @@ def YncaTerminal(serial_url: str):
     With the YNCA terminal you can manually send YNCA commands to a receiver.
     This is useful to figure out what a command does.
 
-    Use ? as <value> to GET the value.
+    Note that a receiver only allows 1 YNCA connection at the time!
+    When connection gets lost immediately after connecting that
+    usually means something else already has a YNCA connection open.
+
     Type 'exit' or 'quit' to exit.
 
+
+    Quick reference:
+
     Command format: @<subunit>:<function>=<value>
+    Use ? as <value> to GET the value.
+
     Examples:
       @SYS:MODELNAME=?
       @MAIN:VOL=-24