Add QStringConverter to string class group

Task-number: QTBUG-77020
Change-Id: Ic11de8f815b7c742761ad9d0884f1eb58534b6ce
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/corelib/text/qstringconverter.cpp

@@ -1971,6 +1971,7 @@ static qsizetype toLatin1Len(qsizetype l) { return l + 1; }
     \brief The QStringConverter class provides a base class for encoding and decoding text.
     \reentrant
     \ingroup i18n
+    \ingroup string-processing
 
     Qt uses UTF-16 to store, draw and manipulate strings. In many
     situations you may wish to deal with data that uses a different
@@ -2705,6 +2706,7 @@ const char *QStringConverter::nameForEncoding(QStringConverter::Encoding e)
     \brief The QStringEncoder class provides a state-based encoder for text.
     \reentrant
     \ingroup i18n
+    \ingroup string-processing
 
     A text encoder converts text from Qt's internal representation into an encoded
     text format using a specific encoding.
@@ -2799,6 +2801,7 @@ const char *QStringConverter::nameForEncoding(QStringConverter::Encoding e)
     \brief The QStringDecoder class provides a state-based decoder for text.
     \reentrant
     \ingroup i18n
+    \ingroup string-processing
 
     A text decoder converts text an encoded text format that uses a specific encoding
     into Qt's internal representation.