Fix documentation about datetime_unit of xarray.DataArray.differentiate

[dcherian]

Should say that Y and M cannot be supported with datetime64

Discussed in #8000

^{Originally posted by jesieleo July 19, 2023}
I have a piece of data that looks like this

<xarray.Dataset>
Dimensions:    (time: 612, LEV: 15, latitude: 20, longitude: 357)
Coordinates:
  * time       (time) datetime64[ns] 1960-01-15 1960-02-15 ... 2010-12-15
  * LEV        (LEV) float64 5.01 15.07 25.28 35.76 ... 149.0 171.4 197.8 229.5
  * latitude   (latitude) float64 -4.75 -4.25 -3.75 -3.25 ... 3.75 4.25 4.75
  * longitude  (longitude) float64 114.2 114.8 115.2 115.8 ... 291.2 291.8 292.2
Data variables:
    u          (time, LEV, latitude, longitude) float32 ...
Attributes: (12/30)
    cdm_data_type:              Grid
    Conventions:                COARDS, CF-1.6, ACDD-1.3
    creator_email:              [email protected]
    creator_name:               APDRC
    creator_type:               institution
    creator_url:                https://www.atmos.umd.edu/~ocean/
    ...                         ...
    standard_name_vocabulary:   CF Standard Name Table v29
    summary:                    Simple Ocean Data Assimilation (SODA) soda po...
    time_coverage_end:          2010-12-15T00:00:00Z
    time_coverage_start:        1983-01-15T00:00:00Z
    title:                      SODA soda pop2.2.4 [TIME][LEV][LAT][LON]
    Westernmost_Easting:        118.25

when i try to use xarray.DataArray.differentiate
data.u.differentiate('time',datetime_unit='M')
will appear

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\Anaconda3\lib\site-packages\xarray\core\dataarray.py", line 3609, in differentiate
    ds = self._to_temp_dataset().differentiate(coord, edge_order, datetime_unit)
  File "D:\Anaconda3\lib\site-packages\xarray\core\dataset.py", line 6372, in differentiate
    coord_var = coord_var._to_numeric(datetime_unit=datetime_unit)
  File "D:\Anaconda3\lib\site-packages\xarray\core\variable.py", line 2428, in _to_numeric
    numeric_array = duck_array_ops.datetime_to_numeric(
  File "D:\Anaconda3\lib\site-packages\xarray\core\duck_array_ops.py", line 466, in datetime_to_numeric
    array = array / np.timedelta64(1, datetime_unit)
TypeError: Cannot get a common metadata divisor for Numpy datatime metadata [ns] and [M] because they have incompatible nonlinear base time units.

Would you please told me is this a BUG?