Discrepancy in DateOffset & Timedelta string representation
#### Code Sample, a copy-pastable example if possible

```python
# DateOffset creation and string representation
off = pd.tseries.frequencies.to_offset('2MS')
off.freqstr
>>> Out[20]: '2MS'

# Timedelta creation from DateOffset string representation
td = pd.to_timedelta(off.freqstr)
td
>>> Out[21] Timedelta('0 days 00:00:00.002000')

# Same for month end 'M'
```
#### Problem description

Hello
I am aware Timedelta & DateOffset objects are not supposed to have same representation, but could their string aliases be different when they cannot be mapped to each other?

For Timedelta / minute vs DateOffset / month start ('M'):
- I understand that representations should be 'min' or 'T' so I am surprised 'M' is accepted by to_timedelta(). To be removed?

For DateOffset / month start vs Timedelta / millisecond ('MS'):
- maybe month start can be replaced by MB 'Month Begin' ? 

Would this make sense?
Bests,