Doc: a QScreen can be a placeholder if there are no screens attached

As of change a094af0017, X11 now joins
other platforms in having a fake screen if no physical screens are
attached, rather than ever allowing QGuiApplication::screens() to
return an empty list or QGuiApplication::primaryScreen() to return
null.  This is more consistent across platforms and helps to prevent
bugs in applications which want to use QScreen in some context but
fail to check whether it exists.

[ChangeLog][X11] In case there are no physical screens attached,
QGuiApplication::screens() and QGuiApplication::primaryScreen() will
return a placeholder QScreen object.

Task-number: QTBUG-42985
Change-Id: I0f58afbfe2a034fb9e872a5d1dbdabad2c7dbf9e
Reviewed-by: Błażej Szczygieł <spaz16@wp.pl>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>

src/gui/kernel/qguiapplication.cpp

@@ -921,15 +921,10 @@ QList<QScreen *> QGuiApplication::screens()
 /*!
     \property QGuiApplication::primaryScreen
 
-    \brief the primary (or default) screen of the application, or null if there is none.
+    \brief the primary (or default) screen of the application.
 
     This will be the screen where QWindows are initially shown, unless otherwise specified.
 
-    On some platforms, it may be null when there are actually no screens connected.
-    It is not possible to start a new QGuiApplication while there are no screens.
-    Applications which were running at the time the primary screen was removed
-    will stop rendering graphics until one or more screens are restored.
-
     The primaryScreenChanged signal was introduced in Qt 5.6.
 
     \sa screens()

src/widgets/kernel/qdesktopwidget.qdoc

@@ -238,11 +238,6 @@
     \property QDesktopWidget::screenCount
     \brief the number of screens currently available on the system.
 
-    Note that on some platforms, screenCount will be zero if there are actually
-    no screens connected. Applications which were running at the time the
-    screenCount went to zero will stop rendering graphics until one or more
-    screens are restored.
-
     \since 4.6
 */