Improve Event callback docs

[brain-geek]

Changing a few things there:

• typo in function name (after_send is not a function)
• Adding a note about how to stop Sentry from reporting event. Filtering Exceptions section says:

If you would like to prevent Sentry from sending certain exceptions, you can use the :before_send configuration option. See the Event Callbacks section below.

But in the Event Callbacks section it does not say a word on how to filter those.

• Adding a type spec for a bit easier investigation into how to test it. I had to look into the code of this actual repo to understand this and previous point. Now it should be a bit more obvious.

#skip-changelog

[whatyouhide]

Can we instead change the function name in the code example to be :after_send?

[brain-geek]

@whatyouhide sure, changed that.