Document different pathnames for articles string formats

[mpacer]

Regarding the pathname formats described here:

https://github.com/mpacer/allofplos/blob/e8a9aee1c5eb587936cdf922f7ccb3fd01f10dc8/allofplos/plos_regex.py#L44-L53

We should be better about describing that, specifically:

We could also use a formal spec in the docs that says what the actual different formats are… I don't know if we should really have a single pair of examples here as opposed to a few examples in its own doc page about said spec.
~ @mpacer #70 (comment)

This should probably happen after the initial Docs PR (#66) is merged.