This fixes#1247. It also regenerates the usage documentation, so that
styling fixex in that section (as well as other existing changes) make
it into the files in docs/.
- Group options
- Nicer list of options in Sphinx
- Deduplicate 'Common options'
(including --help)
The latter is done by explicitly declaring --help in the common_parser,
which is then inherited by the sub-parsers; no change in observable
behaviour.
the generation of those files was causing us way too much pain to
justify automatically generating them all the time.
those will have to be re-generated with `build_api` or `build_usage`
as appropriate, for example when function signatures or commandline
flags change.
see #384
