BUG: Inconsistency in `pd.to_datetime` with `orig` parameter according to docs.
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
>>> pd.to_datetime([1, 2, 3])
DatetimeIndex(['1970-01-01 00:00:00.000000001',
               '1970-01-01 00:00:00.000000002',
               '1970-01-01 00:00:00.000000003'],
              dtype='datetime64[ns]', freq=None)
>>> pd.to_datetime([1, 2, 3], unit='D') # Expected
DatetimeIndex(['1970-01-02', '1970-01-03', '1970-01-04'], dtype='datetime64[ns]', freq=None)
>>> pd.to_datetime(['1', '2', '3'], unit='D') # Expected
DatetimeIndex(['1970-01-02', '1970-01-03', '1970-01-04'], dtype='datetime64[ns]', freq=None)
>>> pd.to_datetime(['1', '2', '3'], format='%d') # Inconsistent, was expecting this to be equivalent of above.
DatetimeIndex(['1900-01-01', '1900-01-02', '1900-01-03'], dtype='datetime64[ns]', freq=None)

```

#### Problem description

According to the `pd.to_datetime` docs, the default `orig` parameter is `unix` where the time starts at `1970-01-01`.
In which case I would be expecting `pd.to_datetime(['1', '2', '3'], format='%d')` to return `1970` years dates instead of treating `1900` as origin.



#### Expected Output

```python
>>> pd.to_datetime(['1', '2', '3'], unit='D') 
DatetimeIndex(['1970-01-02', '1970-01-03', '1970-01-04'], dtype='datetime64[ns]', freq=None)
>>> pd.to_datetime(['1', '2', '3'], format='%d') 
DatetimeIndex(['1970-01-02', '1970-01-03', '1970-01-04'], dtype='datetime64[ns]', freq=None)
```


#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.7.3.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : en_US.UTF-8
pandas           : 1.1.0
numpy            : 1.19.0
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 49.1.0
Cython           : None
pytest           : None
hypothesis       : 5.29.0
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
