Docs change update with show_context = True in .coveragerc

[dkotschessa]

After much googling, I came across this issue, which solved my problem, but which I believe still needs to be documented. Namely, adding

[html]
show_context = True

Will allow context reporting in the html report without having to run coverage separately.

The issue is still open because @brutledge was still discussing whether to make pytest-cov do this automatically.

Feel free to make a PR with the docs change.

Originally posted by @ionelmc in #371 (comment)