Document XCODE variable

Help/manual/cmake-variables.7.rst

@@ -214,6 +214,7 @@ Variables that Describe the System
    /variable/WINCE
    /variable/WINDOWS_PHONE
    /variable/WINDOWS_STORE
+   /variable/XCODE
    /variable/XCODE_VERSION
 
 Variables that Control the Build

Help/variable/XCODE.rst

@@ -0,0 +1,4 @@
+XCODE
+-----
+
+``True`` when using :generator:`Xcode` generator.