DOC: expand doc for numeric_only
- [x] closes #10480
- [x] tests not needed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry not needed

Just making the description (hopefully) a little clearer.  

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14309?src=pr) is 85.26% (diff: 100%)

> Merging [#14309](https://codecov.io/gh/pydata/pandas/pull/14309?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14309   diff @@
==========================================
  Files           140        140          
  Lines         50614      50614          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43155      43156     +1   
+ Misses         7459       7458     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [df50e88...c9657fc](https://codecov.io/gh/pydata/pandas/compare/df50e88f6da84a874402285836483cafa3415292...c9657fca25db5f2daae61eefa13caf740ab339ec?src=pr)
 thanks!
