Sessions: add existing feature documentation

Change-Id: I8504a2fae4476d5d54dc7bdb681a8c9c2c39caaa
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

doc/src/howto/creator-cli.qdoc

@@ -211,6 +211,13 @@
                 \QC. For more information about managing sessions, see
                 \l{Managing Sessions}.
 
+        \row
+            \li <session>
+            \li ProjectExplorer plugin: load the given session when \QC starts.
+                Open the projects and files that were open when you last exited
+                \QC. For more information about managing sessions, see
+                \l{Managing Sessions}.
+
     \endtable
 
     \section1 Using Custom Styles

src/plugins/projectexplorer/ProjectExplorer.json.in

@@ -23,6 +23,10 @@
         {
             \"Name\" : \"-lastsession\",
             \"Description\" : \"Restore the last session\"
+        },
+        {
+            \"Name\" : \"<session>\",
+            \"Description\" : \"Restore a saved session\"
         }
     ],
     $$dependencyList