qdoc: Let default output subdir be html

When you want qdoc to output the docs in a
single output subdirectory, you would add
these lines to your qdocconf file:

HTML.nosubdirs            = "true"
HTML.outputsubdir         = "html"

The name of the output subdir can be anything.
But if you leave out the second line, qdoc now
defaults to using "html" as the single output
subdir.

Task-number: QTBUG-32580
Change-Id: Ibfb2a0c578515ef934e816b2d7a516b64f0f9dcf
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>