Fix a couple documentation punctuation errors

I think it's important to explicitly note that Jekyll ignores the return value of `generate`. Other than that, the verbiage about not "expect[ing] any particular side effect to happen" is kind of confusing—I'm not sure what the original author meant by that. It seems kind of self-explanatory that a method would be run for its effects—in fact, "*side* effect" is probably misleading—so I actually think it makes more sense to take the first sentence and group it with the previous paragraph, and then remove the second sentence:

> ...
>
> A generator is a subclass of `Jekyll::Generator` that defines a `generate` method, which receives an instance of `Jekyll::Site`. The return value of `generate` is ignored.
>
> Generators run after Jekyll has made an inventory ...