add_test: Mention generator expressions in old-style add_test docs

The old-style add_test() call does not support generator expressions at
all. This also applies to the properties for the test, but it is not
mentioned at all.

Source/cmAddTestCommand.h

@@ -66,7 +66,8 @@ public:
       "built by this project or an arbitrary executable on the "
       "system (like tclsh).  The test will be run with the current working "
       "directory set to the CMakeList.txt files corresponding directory "
-      "in the binary tree.\n"
+      "in the binary tree.  Tests added using this signature do not support "
+      "generator expressions.\n"
       "\n"
       "  add_test(NAME <name> [CONFIGURATIONS [Debug|Release|...]]\n"
       "           [WORKING_DIRECTORY dir]\n"