[FR] rework auto-generated docstrings

[t-bltg]

While working on #4503.

Currently, taking the fillrange keyword argument in a docstring:

  Arguments
  ≡≡≡≡≡≡≡≡≡≡≡

    •  fillrange: Number or AbstractVector. Fills area between fillrange and y for line-types, sets the base for
       bar/stick types, and similar for other types.

I think the auto-generated docstrings would benefit from:

• adding the allowed type(s) as an Union suffix (in the form of fillrange::Union{Number,AbstractVector}: <description>);
• add aliases list at the end of the description (fillrange <description> aliases = (fillrng, frange, fillto, fill_between));
• enhance function descriptions in general (src/shorthands.jl).

TBD.