Speedup find_* commands (#11412)

Delay computation of the command documentation until it is needed.
It is wasteful to do it in the constructor on every call.

Inspired-By: Christian Ehrlicher <Ch.Ehrlicher@gmx.de>

Source/cmFindLibraryCommand.h

@@ -64,6 +64,7 @@ protected:
   void AddArchitecturePaths(const char* suffix);
   void AddLib64Paths();
   std::string FindLibrary();
+  virtual void GenerateDocumentation();
 private:
   std::string FindNormalLibrary();
   std::string FindFrameworkLibrary();