DOC: Fix docstring of _median_nancheck.

_median_nancheck doesn't support axis being anything other than an
integer (otherwise the call to moveaxis would fail).  This is fine,
because median goes through _ureduce for multi-axis support.

This change speeds up taking the median of 1001 floats by ~20%, as
measured by
`python -mtimeit -s 'import numpy as np; x = np.random.randn(1001)' -- 'np.median(x)'`

* Add doc for deprecate_with_doc

Co-authored-by: Bas van Beek <43369155+BvB93@users.noreply.github.com>
Co-authored-by: Matti Picus <matti.picus@gmail.com>

Fixes the incompatible type comparison found in #17490.

This also corrects the logic to not print the heading when only private/magic methods are present.

BUG: Indentation for docstrings

Using inspect.signature instead of `np.compat.getargspec` solves this problem.

`inspect.signature` also handles stripping the `self` argument of methods for us.

`global` is only needed if a variable appears on the left of an assignment.
These variables do not.

Most suffer from the misconception that `var[x] = y` requires `var` to be global, but it does not.

More sys.version cleanup.