Fixed label formatting in Settings dialog.

Utkarsh Ayachitrequested to merge
utkarsh.ayachit/paraview:fix_settings_dialog_formatting into master

This bug was introduced by d4193897. Reverted the change to add HTML headers in pqProxy::rstToHtml(). Instead, adding the formatting headers in pqProxyWidget when using the text returned from pqProxyWidget::documentationText() -- as seems to be the norm in the current implementation.

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