docs: Tidy up doctest options section
docs: Tidy up doctest options section

* Move the parts about "how to configure it" (pytest.ini vs. inline
  comment) together.
* Move `--doctest-continue-on-failure` into its own sub-heading, as it
  isn't related to the doctest optionflags.

nicoddemus/pytest,tomviner/pytest,The-Compiler/pytest,nicoddemus/pytest,alfredodeza/pytest,markshao/pytest,RonnyPfannschmidt/pytest,pytest-dev/pytest,tomviner/pytest,The-Compiler/pytest,Akasurde/pytest