pd.Timedelta.resolution is different from datetime.timedelta.resolution
`timedelta.resolution` is documented as:
> The smallest possible difference between non-equal timedelta objects, timedelta(microseconds=1).

while `pandas.Timedelta.resolution` is doc-string'd as:
>return a string representing the lowest resolution that we have
```
In [9]: pd.Timedelta(minutes=30).resolution
Out[9]: 'T'

In [10]: timedelta(minutes=30).resolution
Out[10]: datetime.timedelta(0, 0, 1)
```

I think this difference should be addressed by either:
1) Documenting the difference between both but also supporting `datetime.timedelta`'s notion of resolution in `Timedelta` (i.e. something that returns `Timedelta(nanoseconds=1)`)
2) Making `Timedelta.resolution` consistent with `timedelta.resolution` and creating a new property for "the lowest resolution" of a `Timedelta` instance

Thoughts? 

xref #21336
