Add comment to a tr() call

Add a comment for the translator explaining what the "+" sign is
being used for that they are translating.

Fixes: QTBUG-2657
Change-Id: I4c828bcc36fa94c0f1771831667b2b1f591ae92e
Reviewed-by: Lucie Gerard <lucie.gerard@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/gui/kernel/qkeysequence.cpp

@@ -1221,10 +1221,12 @@ QString QKeySequence::encodeString(int key)
 static inline void addKey(QString &str, const QString &theKey, QKeySequence::SequenceFormat format)
 {
     if (!str.isEmpty()) {
-        if (format == QKeySequence::NativeText)
+        if (format == QKeySequence::NativeText) {
+            //: Key separator in shortcut string
             str += QCoreApplication::translate("QShortcut", "+");
-        else
+        } else {
             str += QLatin1Char('+');
+        }
     }
 
     str += theKey;