Doc: add note about placeholder text to Material style documentation

Also update an old link.

Task-number: QTBUG-117654
Pick-to: 6.5
Change-Id: Ib0c49052e15c7eca5dd6197ddbc081fb483a74d1
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
(cherry picked from commit 8f876c620c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 51c6cac7e4)

src/quickcontrols/doc/src/includes/qquickmaterialstyle.qdocinc

@@ -77,3 +77,8 @@
             for example \c "Grey".
 \endtable
 //! [env]
+
+//! [placeholder-text-multiple-lines]
+As per the Material guidelines, placeholder text should be kept short and not
+take up multiple lines.
+//! [placeholder-text-multiple-lines]

src/quickcontrols/doc/src/qtquickcontrols-material.qdoc

@@ -32,7 +32,7 @@
     \section1 Detailed Description
     \target detailed-desc-material
 
-    The Material style is based on the \l {https://www.google.com/design/spec/material-design/introduction.html}
+    The Material style is based on the \l {https://m3.material.io}
     {Google Material Design Guidelines}. It allows for a unified experience
     across platforms and device sizes.
 
@@ -259,6 +259,8 @@
     \l {Item::}{clip} to \c true. To avoid this, \l {Control::}{topInset} is
     set to an appropriate value in these cases.
 
+    \include qquickmaterialstyle.qdocinc placeholder-text-multiple-lines
+
     \section3 TextField
 
     The same \l {material-control-specific-notes-textarea}{issue with clipping}
@@ -266,6 +268,8 @@
     this, \l {Control::}{topInset} is set to an appropriate value when the
     TextField sets clip to \c true.
 
+    \include qquickmaterialstyle.qdocinc placeholder-text-multiple-lines
+
     \section1 Attached Property Documentation
 
     \styleproperty {Material.accent} {color} {material-accent-attached-prop}