Doc: Documentation for QStringList::replaceInStrings

Added the text as a \note in QString::replace and QStringlist::replace
to make it more visible to the users.

Fixes: QTBUG-63485
Pick-to: 6.7
Change-Id: I519ab079ec86103a768f313cbc5ac625a0170839
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 3e56757d651ffcba9ea65aa990b4e02b05e00b17)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qstring.cpp

@@ -3786,6 +3786,9 @@ static void replace_helper(QString &str, QSpan<size_t> indices, qsizetype blen,
   but \a position + \a n goes outside the strings range,
   then \a n will be adjusted to stop at the end of the string.
 
+  \note If you use an empty \a before argument, the \a after argument will be
+  inserted \e {before and after} each character of the string.
+
   Example:
 
   \snippet qstring/main.cpp 40

src/corelib/text/qstringlist.cpp

@@ -382,6 +382,9 @@ QStringList QtPrivate::QStringList_filter(const QStringList *that, const QRegula
     text replaced with the \a after text wherever the \a before text
     is found.
 
+    \note If you use an empty \a before argument, the \a after argument will be
+    inserted \e {before and after} each character of the string.
+
     \include qstringlist.cpp comparison-case-sensitivity
 
     For example: