QString: add note about isNull() vs. isEmpty() to more methods

Namely compare() and operator==().

Change-Id: I7e404f2f1df5b93614f7e266bdbebe909e4e7623
Fixes: QTBUG-54505
Pick-to: 6.5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qstring.cpp

@@ -3907,6 +3907,8 @@ QString &QString::replace(QChar c, QLatin1StringView after, Qt::CaseSensitivity
     Returns \c true if string \a s1 is equal to string \a s2; otherwise
     returns \c false.
 
+    \include qstring.cpp compare-isNull-vs-isEmpty
+
     \sa {Comparing Strings}
 */
 
@@ -6295,6 +6297,11 @@ QString& QString::fill(QChar ch, qsizetype size)
 
     \snippet qstring/main.cpp 16
 
+//! [compare-isNull-vs-isEmpty]
+    \note This function treats null strings the same as empty strings,
+    for more details see \l {Distinction Between Null and Empty Strings}.
+//! [compare-isNull-vs-isEmpty]
+
     \sa operator==(), operator<(), operator>(), {Comparing Strings}
 */