Add some comments to VT state machine

For clarity, the VT state machine has some comments where it differs
from the diagram from vt100.net. Add comments for some transitions on
String Terminator (ST / 0x9C) which appear on the diagram, but which
were already handled in the "anywhere" state.

src/Vt102Emulation.cpp

@@ -868,11 +868,13 @@ void Vt102Emulation::receiveChars(const QVector<uint> &chars)
                 case DcsPassthrough:
                     if (cc <= 0x7E || cc >= 0xA0) { // 0x18, 0x1A, 0x1B already taken care of
                         put(cc);
+                    // 0x9C already taken care of.
                     } else if (cc == 0x7F) {
                         // ignore
                     }
                     break;
                 case DcsIgnore:
+                    // 0x9C already taken care of.
                     if (cc <= 0x7F) {
                         // ignore
                     }
@@ -889,8 +891,9 @@ void Vt102Emulation::receiveChars(const QVector<uint> &chars)
                     break;
                 case SosPmApcString:
                     if (cc <= 0x7F || cc >= 0xA0) { // 0x18, 0x1A, 0x1B already taken care of.
-                        apc_put(cc);
+                        apc_put(cc); // while the vt100.net diagram has ignore here, konsole does process some APCs (kitty images).
                     }
+                    // 0x9C already taken care of.
                     break;
                 default:
                     break;