Chart dual axis: DHX-1869 - #157

- the type of the horizontal Bar chart is "xbar", not "x-bar": the value
  was renamed in the widget before Suite 6.0 and the pages kept the old
  spelling since the docs import
- corrected in the type, series, scales and legend API pages and in the
  configuration guide
- the historical whatsnew entry is left as is

- new sections on the series and scales API pages that hold the
  descriptions of scale, scales, stacked, grid and alignTicks, with the
  signature, the rendering cases, an example and the related links of
  every property
- the rows of the option tables shortened to a line pointing at the
  corresponding section
- the dual axis guide section links to the API pages of the properties
  it mentions, and every new API section links back to the guide
- the grid sample of the guide matches the sentence it follows, and the
  lead of the stack groups describes the whole subsection
- the mixing paragraph no longer contradicts the note on Bar and X-Bar
- the scale sample of the series page and the container ids of the guide
  samples brought in line with the rest of the documentation

- four screenshots of the dual axis section: the value scale of a series,
  the aligned ticks, the stack of every scale and the named stack groups
- highlighted lines in the samples of the new properties
- the scale positions are written as code, and the value lists of
  alignTicks and stacked follow one shape on the guide and the API pages
- a definition of the direction of a scale on all the three pages, and a
  section of its own for the default and the incorrect scale positions
- the samples and the images agree on the titles of the scales
- the note on Bar and X-Bar moved to the mixed graphs section, where the
  limitation belongs

- the table rows of grid and stacked carry their general description and
  their sample again, and point at the section for the dual axis case
- the sections of grid and stacked hold the dual axis part only
- what the second scale of a direction renders is described once, in the
  alignTicks value list, as the specification puts it
- the intro of the dual axis options no longer repeats the wording of the
  scales row