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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a