Xcode: Teach XCODE_ATTRIBUTE target properties about generator expressions

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>

Help/prop_tgt/XCODE_ATTRIBUTE_an-attribute.rst

@@ -8,3 +8,9 @@ the generated Xcode project.  Ignored on other generators.
 
 See the :variable:`CMAKE_XCODE_ATTRIBUTE_<an-attribute>` variable
 to set attributes on all targets in a directory tree.
+
+Contents of ``XCODE_ATTRIBUTE_<an-attribute>`` may use
+"generator expressions" with the syntax ``$<...>``.  See the
+:manual:`cmake-generator-expressions(7)` manual for available
+expressions.  See the :manual:`cmake-buildsystem(7)` manual
+for more on defining buildsystem properties.

Help/release/dev/xcode-attribute-genex.rst

@@ -0,0 +1,5 @@
+xcode-attribute-genex
+---------------------
+
+* The :prop_tgt:`XCODE_ATTRIBUTE_<an-attribute>` target property learned
+  to support generator expressions.