MNT/TST: remove xcorr and acorr from test_datetime

[jklymak]

Currently xcorr and acorr are in test_datetimes.py. However, I think it is highly unlikely anyone would want to pass datetimes in as x values to either of these methods. It is also not clear what giving the unit conversion means for these arguments. Modified the docstrings to make this clear.

[story645]

However, I think it is highly unlikely anyone would want to pass datetimes in as x values to either of these methods

If we claim to support it, i.e. this participates in units, then it should probably be tested. Also timedeltas are common for these https://online.stat.psu.edu/stat462/node/188/

[jklymak]

Xcorr and acorr do not participate in units.

Both work by assuming x and y are spaced equally in time (or space). You can of course assign units to the lags, but we do not try to do this in xcorr/acorr.

[story645]

Xcorr and acorr do not participate in units.

It might be good to generate a table of what participates in units and what doesn't, possibly added either to https://matplotlib.org/devdocs/users/explain/axes/axes_units.html#more-about-unit-support or https://matplotlib.org/devdocs/api/units_api.html

[oscargus]

I agree that these should be removed. Some formatting, although I think that this probably should not go on the same line. Maybe better to add it on the next line? Or as an admonition?

[oscargus]

Suggested change

	x, y : array-like of length n. Neither x nor y are run through
	x, y : array-like of length n. Neither *x* nor *y* are run through