progress: document progress.debug config option

Authored by martinvonz.

Description

progress: document progress.debug config option

I think it was not spotted by test-check-config.t that we had not
documented it because no caller refers to the ui object simply as "ui"
(it was either "self.ui" or just "self").

Differential Revision: https://phab.mercurial-scm.org/D5526

Details