add_test: Document test name restrictions (#14298)

Source/cmAddTestCommand.h

@@ -71,6 +71,9 @@ public:
       "  add_test(NAME <name> [CONFIGURATIONS [Debug|Release|...]]\n"
       "           [WORKING_DIRECTORY dir]\n"
       "           COMMAND <command> [arg1 [arg2 ...]])\n"
+      "Add a test called <name>.  "
+      "The test name may not contain spaces, quotes, or other characters "
+      "special in CMake syntax.  "
       "If COMMAND specifies an executable target (created by "
       "add_executable) it will automatically be replaced by the location "
       "of the executable created at build time.  "