angular_diameter_distance documents z1 while the parameter is z #20206

Following up on #20205, where @pllim asked me to open one issue per remaining problem.

Cosmology.angular_diameter_distance heads its Parameters section with z1, z2:

def angular_diameter_distance(self, z: _InputT, z2: _InputT | None = None, /) -> u.Quantity:
    """...

    Parameters
    ----------
    z1, z2 : Quantity-like ['redshift'], array-like
        Input redshifts. If one argument ``z`` is given, the distance :math:`d_A(0, z)`
        is returned. If two arguments ``z1, z2`` are given, ...

The first parameter is z, not z1. Since both are positional-only, nothing breaks at call time, but the rendered signature and the section heading disagree, and numpydoc-style tooling that matches names will not pair the heading with the parameter.

I did not change it in the PR because the prose underneath deliberately explains the one-argument and two-argument forms in terms of z, z1 and z2, so renaming the heading to z, z2 may lose that. Two options I can see:

• head the section z, z2 and keep the existing prose, which already spells out that a single argument is z;
• leave it as is, if the current wording is considered clearer than the name match.

Happy to send a PR for either, or to leave it alone.

Found with a static checker comparing numpydoc parameter names against the signature below them; each finding was read by hand.