[docs] Record how the contributors are expected to handle Cog failures

[webknjaz]

It seems like tox has cogapp integrated in a way that it does not give a contributor action items to follow. It just prints out Check failed and unless one digs deep to understand the connection between RST->tox->cog->cog syntax->standalone program install, it might be difficult for a newbie to proceed with reacting to the failure.

It's not mentioned anywhere in the contributing doc (#2051). The tox-integrated commands always have --check with no integration for the caller to replace it with other args.

I imagine the command to document would be

$ tox exec -q -e doc -- python -m cogapp -cr 'doc/*.rst'`

But in general, I think the entire expected workflow should be documented somehow.

Another possibility would be implementing nedbat/cog#21 and moving it there in a normal formatting mode.

[webknjaz]

Oh, I see there's a make cogdoc that might work. It just needs to be documented.

[nedbat]

Commit c181b93 uses a new cog option to provide instructions if a check fails, and
commit 04bbc3a adds details to the contributing docs.

As it happens, some of the wiring was wrong, so this was good to work through!