Fix --tag documentation to avoid confusing "filter" references.

Created by: creature

As reported in #2017 (closed), the sentence fragment "then foo will be filtered" usually means that "foo" will be filtered out. The tag feature actually works the opposite way; "--tag foo" will only run examples with foo: true.

I've tried to tidy up this documentation to make this clearer, as well as moving the paragraph about internal details lower down so exclusions are discussed before we use them in an example.