re #12069

re #11821

Including
1. Add support of default fit windows to FitwindowTableWorkspace, as
well as the unit test;
2. Improve the doc.

As Martyn's suggestion.  Though it does not seems to change any format
in the generated html file.
On the other hand, the previously reported warning was fixed by last
commit (http://builds.mantidproject.org/job/develop_clean/505/)

The first underscore is not included in the reference name.
Refs #9562

Uses # to denote a html h3 heading.
Refs #9562

The converted text is stored in separate .rst files in the docs directory.
Refs #9562