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/cmFindPackageCommand.h

@@ -65,6 +65,8 @@ public:
   virtual const char* GetFullDocumentation();
 
   cmTypeMacro(cmFindPackageCommand, cmFindCommon);
+protected:
+  virtual void GenerateDocumentation();
 private:
   void AppendSuccessInformation();
   void AppendToProperty(const char* propertyName);