cmDocumentationEntry: Drop all user provided ctors for C++ >= 14

There is no need for them cuz:

- the last field has a default value
- all static instances use 2 arguments convertible to `std::string`
- "dynamic" instances used for _Generator_ doc entries access
  fields diectly using default constructed instance

Moreover, compiler may generate move ctor/assign when needed.

Source/CPack/cpack.cxx

@@ -36,11 +36,11 @@
 
 namespace {
 const cmDocumentationEntry cmDocumentationName = {
-  nullptr, "  cpack - Packaging driver provided by CMake."
+  {},
+  "  cpack - Packaging driver provided by CMake."
 };
 
-const cmDocumentationEntry cmDocumentationUsage = { nullptr,
-                                                    "  cpack [options]" };
+const cmDocumentationEntry cmDocumentationUsage = { {}, "  cpack [options]" };
 
 const cmDocumentationEntry cmDocumentationOptions[14] = {
   { "-G <generators>", "Override/define CPACK_GENERATOR" },