Document pytest_report_header and conftest behavior

[vmuriart]

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs:

• Target: for bug or doc fixes, target master; for new features, target features;

Unless your change is trivial documentation fix (e.g., a typo or reword of a small section) please:

• Make sure to include one or more tests for your change;
• Add yourself to AUTHORS;
• Add a new entry to CHANGELOG.rst
  • Choose any open position to avoid merge conflicts with other PRs.
  • Add a link to the issue you are fixing (if any) using RST syntax.
  • The pytest team likes to have people to acknowledged in the CHANGELOG, so please add a thank note to yourself ("Thanks @user for the PR") and a link to your GitHub profile. It may sound weird thanking yourself, but otherwise a maintainer would have to do it manually before or after merging instead of just using GitHub's merge button. This makes it easier on the maintainers to merge PRs.

[coveralls]

Coverage remained the same at 92.709% when pulling a88017c on vmuriart:doc-report_header into 9c809f5 on pytest-dev:master.

[nicoddemus]

Excellent, many thanks!