Doc: Make it explicit what QJsonParseError::offset refers to

Pick-to: 6.7 6.5 6.2
Change-Id: I278929f89e9e6f9f4461558dc0dd949d0455dcb1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit e00a032eac1a234c0846571a0e8a6ac1e5c411b7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/serialization/qjsonparser.cpp

@@ -102,9 +102,9 @@ using namespace QtMiscUtils;
 /*!
     \variable QJsonParseError::offset
 
-    Contains the offset in the input string where the parse error occurred.
+    Contains the byte offset in the UTF-8 byte array where the parse error occurred.
 
-    \sa error, errorString()
+    \sa error, errorString(), QJsonDocument::fromJson()
 */
 
 /*!