content: add info about the ripple effect duration

_Original issue by @distante on [2020-08-16T08:46:00Z](https://github.com/ionic-team/ionic-framework/issues/21922)_

# Documentation

The ripple effect duration used in components like `ion-button` is not documented. It would be useful for some developers that want to trigger some animation orchestration starting with a button click.

---

I can do a PR for this but I am not sure which duration this effect has. From the [source code I would say something like `325ms`](https://github.com/ionic-team/ionic-framework/blob/master/core/src/components/ripple-effect/ripple-effect.tsx#L66) but I am not really sure.
```
setTimeout(() => {
resolve(() => {
removeRipple(div);
});
}, 225 + 100);
```

Extra: what does that 225 and 100 mean?

Start with core/src/components/ripple-effect/ripple-effect.tsx at the linked line and inspect how the 225 and 100 millisecond values are used. Confirm the effective ripple duration and what each value represents, then update the relevant Ionic documentation. Done means developers can find the duration and understand the timing values.

索引モデルが issue の本文から書いたものです。