General grammar and style fixes in the book

Contains numerous grammar, style and readability fixes (see diff)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >= 2.3
| Fixed tickets | 
Some of the "or not"'s removed in this PR make sense, because they don't really add anything. But I think this one does - it highlights that we're looking for an positive or negative message distinctly.

We could alternatively use something like this (though I don't have a problem with the "or not" - but if you know that this is not easily understandable by non-native speakers or perhaps just wrong grammatically, let me know):

>
the cache asks the application if the cached response is still valid or if it needs to be regenerated.