DOC:updated docstring in .to_datetime()
- [x] closes #14448 
- [ ] tests added / passed
- [ ] passes git diff upstream/master | flake8 --diff
- [ ] whatsnew entry

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14452?src=pr) is 85.25% (diff: 100%)

> Merging [#14452](https://codecov.io/gh/pandas-dev/pandas/pull/14452?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14452   diff @@
==========================================
  Files           140        140          
  Lines         50631      50655    +24   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43166      43185    +19   
- Misses         7465       7470     +5   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [c31ea34...3aa78cf](https://codecov.io/gh/pandas-dev/pandas/compare/c31ea341747ac40d83e64d00bac179548e4b716e...3aa78cf021ab0ca3680a0d42a9796c6e4a86b425?src=pr)
 - The [timestamp limitation](http://pandas-docs.github.io/pandas-docs-travis/timeseries.html#timestamp-limitations) is that "the timespan that can be represented using a 64-bit integer is limited to approximately 584 years". 
- So the default behavior (from version 0.17.0) is `errors='raise'` i.e. when a date does not meet timestamp limitations, an exception will be raised. 
  Setting `errors='ignore'` will simply return the original input instead of raising any exception.
 @sinhrks Can this be merged?
 @jreback Can this be merged? Please let me know if there are any changes to be added.
 thanks!
