Doc: use const& in foreach when applicable.

Const reference are slightly faster than doing a copy of the object
on each iteration.

Lead by example by having this pattern right in our documentation

1 files changed, 1 insertions, 1 deletions

diff --git a/doc/src/snippets/qstringlist/main.cpp b/doc/src/snippets/qstringlist/main.cpp
index 76785f1..9ada319 100644
--- a/doc/src/snippets/qstringlist/main.cpp
+++ b/doc/src/snippets/qstringlist/main.cpp
@@ -118,7 +118,7 @@ Widget::Widget(QWidget *parent)
 
     result.clear();
 //! [12]
-    foreach (QString str, list) {
+    foreach (const QString &str, list) {
         if (str.contains("Bill"))
             result += str;
     }