DOC: Refactor numeric index docs
Refactor `NumericIndex` docs to avoid duplicate documentation.  xref #14937.
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14985?src=pr) is 84.78% (diff: 100%)
> Merging [#14985](https://codecov.io/gh/pandas-dev/pandas/pull/14985?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14985   diff @@
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  Files           145        145          
  Lines         51090      51107    +17   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits          43315      43333    +18   
+ Misses         7775       7774     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [7f0eefc...ba80766](https://codecov.io/gh/pandas-dev/pandas/compare/7f0eefc2f81d6b34f252a5da38d31e515c295106...ba807666da2628ee9b8258f9b9676ae18761e880?src=pr) nit: needs a blank like before ``Paramaters``. need to wrap the line for Int64Index.

```

In [1]: pd.Int64Index?
Init signature: pd.Int64Index(data=None, dtype=None, copy=False, name=None, fastpath=False)
Docstring:     
Immutable ndarray implementing an ordered, sliceable set. The basic object
storing axis labels for all pandas objects. Int64Index is a special case
of `Index` with purely integer labels. This is the default index type used by the DataFrame and Series ctors when no explicit index is provided by the user.
Parameters
----------
data : array-like (1-dimensional)
dtype : NumPy dtype (default: int64)
copy : bool
    Make a copy of input ndarray
name : object
    Name to be stored in the index
Notes
-----
An Index instance can **only** contain hashable objects.
File:           ~/pandas/pandas/indexes/numeric.py
Type:           type

In [2]: pd.Float64Index?
Init signature: pd.Float64Index(data=None, dtype=None, copy=False, name=None, fastpath=False)
Docstring:     
Immutable ndarray implementing an ordered, sliceable set. The basic object
storing axis labels for all pandas objects. Float64Index is a special case
of `Index` with purely integer labels. 
Parameters
----------
data : array-like (1-dimensional)
dtype : NumPy dtype (default: float64)
copy : bool
    Make a copy of input ndarray
name : object
    Name to be stored in the index
Notes
-----
An Index instance can **only** contain hashable objects.
File:           ~/pandas/pandas/indexes/numeric.py
Type:           type
``` @jreback : Made the requested doc changes.  Ready to merge if there are no other concerns. @jorisvandenbossche : I think sharing docstrings across private functions is fine.  I get more confused by the inconsistencies across written-out docstrings for the same functions, whether private or public.  Not to mention, there are already numerous private functions in `base.py` that docstring-shared. @jreback , @jorisvandenbossche : If there are no other concerns, I think this is good to merge. thanks!