Help: Clarify deprecation of source-file property COMPILE_DEFINITIONS_<CONFIG>

It is not covered by policy `CMP0043`.  Also fix its link
to the `COMPILE_DEFINITIONS` source file property.

Help/prop_sf/COMPILE_DEFINITIONS.rst

@@ -28,5 +28,5 @@ does not support per-config per-source settings, so expressions
 that depend on the build configuration are not allowed with that
 generator.
 
-Generator expressions should be preferred instead of setting the alternative per-configuration
-property.
+Prefer using generator expressions in :prop_sf:`!COMPILE_DEFINITIONS` over the
+deprecated :prop_sf:`COMPILE_DEFINITIONS_<CONFIG>` property.

Help/prop_sf/COMPILE_DEFINITIONS_CONFIG.rst

@@ -1,10 +1,14 @@
 COMPILE_DEFINITIONS_<CONFIG>
 ----------------------------
 
-Ignored.  See CMake Policy :policy:`CMP0043`.
+.. deprecated:: 3.0
+
+  Prefer the :prop_sf:`COMPILE_DEFINITIONS` source-file property with
+  :manual:`generator expressions <cmake-generator-expressions(7)>`.
 
 Per-configuration preprocessor definitions on a source file.
 
-This is the configuration-specific version of :prop_tgt:`COMPILE_DEFINITIONS`.
+This is the configuration-specific version of :prop_sf:`COMPILE_DEFINITIONS`.
+
 Note that :generator:`Xcode` does not support per-configuration source
 file flags so this property will be ignored by the :generator:`Xcode` generator.