Clarify the need for the C extension

[nedbat]

More and more features only work with the C extension.

1. The docs should be explicit about this need.

2. It should be much easier to see if the C extension is available. Perhaps include the info in the output of coverage --version

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/479

[nedbat]

coverage --help and coverage --version now show the tracer class in changeset 6aa08e5f71eb (bb)

[nedbat]

Docs are updated in e000a8e3ccda (bb).