Review documentation

[jpmckinney]

Need to review CLI option documentation (--help) https://open-contracting.github.io/spoonbill/cli.html#reference

We can also review the docstrings (lower priority) https://open-contracting.github.io/spoonbill/api.html

While reviewing the docstrings, we will likely end up reading the code. Anything we learn about the architecture that is worth noting can be added to the docstrings (e.g. the class docstrings can further describe their role in processing data and their relationship to other components).

[jpmckinney]

I had also made a note to try to use Click's show_default configuration to auto-document the defaults.

Noting that #124 added show_default, to avoid writing the defaults in the command's description, and then #125 removed it, because all the tests failed.

Also noting that #124 hardcodes some text like "OCDS 1.1.5" because #123 had used formatted strings instead of named placeholders, as required by gettext.