[clang-tidy] Improve documentation of bugprone-pointer-arithmetic-on-polymorphic-object

[chrchr-github]

https://clang.llvm.org/extra/clang-tidy/checks/bugprone/pointer-arithmetic-on-polymorphic-object.html

• The first example doesn't compile (virtual void ~Base();)
• The example also produces a clang-analyzer-cplusplus.NewDelete warning, which might lead to confusion

A better example could be

struct Base {
  virtual ~Base();
  int i;
};

struct Derived : public Base {};

int foo(const Derived d[]) {
    return d[1].i;
}

In this case, making Derived final suppresses the warning, which should also be mentioned.

[MainakSil]

I can try this.