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

mercurial/help/config.txt

@@ -1756,6 +1756,9 Mercurial commands can draw progress bar
 possible. Some progress bars only offer indeterminate information, while others
 have a definite end point.
 
+``debug``
+    Whether to print debug info when updating the progress bar. (default: False)
+
 ``delay``
     Number of seconds (float) before showing the progress bar. (default: 3)