Xcode: Add documentation for schema generator

Help/manual/cmake-properties.7.rst

@@ -45,6 +45,7 @@ Properties of Global Scope
    /prop_gbl/TARGET_SUPPORTS_SHARED_LIBS
    /prop_gbl/USE_FOLDERS
    /prop_gbl/XCODE_EMIT_EFFECTIVE_PLATFORM_NAME
+   /prop_gbl/XCODE_GENERATE_SCHEME
 
 .. _`Directory Properties`:
 

Help/prop_gbl/XCODE_GENERATE_SCHEME.rst

+XCODE_GENERATE_SCHEME
+---------------------
+
+If enabled, the Xcode generator will generate schema files. Those are
+are useful to invoke analyze, archive, build-for-testing and test
+actions from the command line.
+
+.. note::
+
+  The Xcode Schema Generator is still experimental and subject to
+  change.

Help/release/dev/cmake-xcode-schemes.rst

+cmake-xcode-schemes
+-------------------
+
+* The :generator:`Xcode` generator got the ability to create schema files.
+  This is still an experimental feature and can be activated by setting the
+  :prop_gbl:`XCODE_GENERATE_SCHEME` global property to a ``TRUE`` value.