Disambiguate definitions of "Basic shapes"

[fred-wang]

cc @nikolaszimmermann @rbuis @soniasingla

The introduction of chapter 10 (Basic shapes) starts that way (see https://svgwg.org/svg2-draft/shapes.html#Introduction ):

dl
  dt basic shape
  dt shape
  dt shape elements
  dd A graphics element that is defined by some
  combination of straight lines and curves.
  Specifically: ‘circle’, ‘ellipse’, ‘line’, ‘path’,
  ‘polygon’, ‘polyline’ and ‘rect’.

... more text ...

Presented that way, it looks like "basic shape", "shape" and "shape elements" are all following the same definition, that includes the <path> element.

On the one hand <path> is actually not defined in this chapter and it does not sound appropriate to use the term "basic" for that element since it can be used for quite complex shapes. The text below the definition as well as https://svgwg.org/svg2-draft/paths.html#Introduction seems to exclude <path> for basic shape.

On the other hand, the definition for <path> in https://svgwg.org/svg2-draft/paths.html#PathElement puts it in the category of "shape element".

So it looks that one needs two separate definitions instead : one for basic shape element and one for shape element.