Help: Reference CMAKE_INSTALL_NAME_DIR from INSTALL_NAME_DIR

Help/prop_tgt/INSTALL_NAME_DIR.rst

@@ -6,3 +6,7 @@ Mac OSX directory name for installed targets.
 INSTALL_NAME_DIR is a string specifying the directory portion of the
 "install_name" field of shared libraries on Mac OSX to use in the
 installed targets.
+
+This property is initialized by the value of the variable
+:variable:`CMAKE_INSTALL_NAME_DIR` if it is set when a target is
+created.