Help: Document intended purpose of XCODE_ATTRIBUTE_<an-attribute>

Also warn the reader against setting values CMake normally generates.

Issue: #21728

Help/prop_tgt/XCODE_ATTRIBUTE_an-attribute.rst

@@ -3,9 +3,15 @@ XCODE_ATTRIBUTE_<an-attribute>
 
 Set Xcode target attributes directly.
 
-Tell the :generator:`Xcode` generator to set '<an-attribute>' to a given
+Tell the :generator:`Xcode` generator to set ``<an-attribute>`` to a given
 value in the generated Xcode project.  Ignored on other generators.
 
+This offers low-level control over the generated Xcode project file.
+It is meant as a last resort for specifying settings that CMake does
+not otherwise have a way to control.  Although this can override a
+setting CMake normally produces on its own, doing so bypasses CMake's
+model of the project and can break things.
+
 See the :variable:`CMAKE_XCODE_ATTRIBUTE_<an-attribute>` variable
 to set attributes on all targets in a directory tree.
 

Help/variable/CMAKE_XCODE_ATTRIBUTE_an-attribute.rst

@@ -5,8 +5,14 @@ CMAKE_XCODE_ATTRIBUTE_<an-attribute>
 
 Set Xcode target attributes directly.
 
-Tell the :generator:`Xcode` generator to set '<an-attribute>' to a given value
-in the generated Xcode project.  Ignored on other generators.
+Tell the :generator:`Xcode` generator to set ``<an-attribute>`` to a given
+value in the generated Xcode project.  Ignored on other generators.
+
+This offers low-level control over the generated Xcode project file.
+It is meant as a last resort for specifying settings that CMake does
+not otherwise have a way to control.  Although this can override a
+setting CMake normally produces on its own, doing so bypasses CMake's
+model of the project and can break things.
 
 See the :prop_tgt:`XCODE_ATTRIBUTE_<an-attribute>` target property
 to set attributes on a specific target.