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/QtDialog/CMakeSetup.cxx

@@ -24,11 +24,12 @@
 
 namespace {
 const cmDocumentationEntry cmDocumentationName = {
-  nullptr, "  cmake-gui - CMake GUI."
+  {},
+  "  cmake-gui - CMake GUI."
 };
 
 const cmDocumentationEntry cmDocumentationUsage = {
-  nullptr,
+  {},
   "  cmake-gui [options]\n"
   "  cmake-gui [options] <path-to-source>\n"
   "  cmake-gui [options] <path-to-existing-build>\n"