Doc: Describe settings for opening files and handling open files

Task-number: QTCREATORBUG-21898
Change-Id: I8d6778f858ee8d328819c7377307beb6a9602bc4
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: hjk <hjk@qt.io>

doc/src/howto/creator-sidebar-views.qdoc

@@ -287,6 +287,33 @@
             closed when \uicontrol {Close All} is used.
     \endlist
 
+    To specify settings for opening files and handling open files, select
+    \uicontrol Tools > \uicontrol Options > \uicontrol Environment >
+    \uicontrol System:
+
+    \image qtcreator-options-environment-system.png "Environment options System tab"
+
+    \list
+        \li In the \uicontrol {When files are externally modified} field,
+            select whether you want to be prompted to reload open files
+            that were modified externally. For example, when you pull
+            changes from a version control system.
+        \li Select the \uicontrol {Auto-save modified files} check box to
+            automatically save changed files at the intervals specified in
+            the \uicontrol Interval field.
+        \li Select the \uicontrol {Auto-suspend unmodified files} check
+            box to automatically free the resources of open files after
+            prolonged inactivity. The files are still listed in the
+            \uicontrol {Open Documents} view. Set the minimum number of files
+            that should be kept in memory in the \uicontrol {Files to keep open}
+            field.
+        \li Select the \uicontrol {Warn before opening text files greater than}
+            check box to receive warnings about opening big text files.
+        \li In the \uicontrol {Maximum number of entries in "Recent Files"}
+            field, set the number of recently opened files listed in
+            \uicontrol File > \uicontrol {Recent Files}.
+    \endlist
+
     \section1 Viewing Defined Types and Symbols
 
     The \uicontrol Outline view shows an overview of defined types and other