Use DeprecationWarning for core.common deprecations (GH13634)
Related to second question in #13634 (whether to use FutureWarning or DeprecationWarning in deprecating the public pandas.core.common functions).

As those functions are mostly used in library code, and less directly by users in their own code, I think a DeprecationWarning is more appropriate in this case. 
For example, in our own docs, we started to get warnings due to an example with a statsmodels regression that uses patsy using one of those functions. Note that recent IPython also shows DeprecationWarnings when using a deprecated function interactively.

lgtm
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13990?src=pr) is 85.28% (diff: 100%)

> Merging [#13990](https://codecov.io/gh/pydata/pandas/pull/13990?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13990   diff @@
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  Files           139        139          
  Lines         50206      50206          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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  Hits          42818      42818          
  Misses         7388       7388          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [29d9e24...2de5d48](https://codecov.io/gh/pydata/pandas/compare/29d9e24f4c778b0c9ebe9288bfc217808d2c6edb...2de5d481cba8d7041de96f6f133261855937a666?src=pr)
 thanks @jorisvandenbossche 

I made a slight doc update.
 Thanks!
