DOC: improve cross-links in docs between .expanding and .cum*

[jreback]

xref #12648

• cross-link doc-strings for .cum* to the .expanding().* (for sum,prod,max,min, which are only supported).
• show an example (in computation.rst wher eexpanding is shown? or basics.rst (where cum* are shown)) how these are related (w.r.t. NaN filling).

In [7]: s = Series([1,2,np.nan,3,np.nan,4])

In [8]: s.cumsum()
Out[8]: 
0     1.0
1     3.0
2     NaN
3     6.0
4     NaN
5    10.0
dtype: float64

In [9]: s.expanding().sum()
Out[9]: 
0     1.0
1     3.0
2     3.0
3     6.0
4     6.0
5    10.0
dtype: float64

In [10]: s.cumsum().ffill()
Out[10]: 
0     1.0
1     3.0
2     3.0
3     6.0
4     6.0
5    10.0
dtype: float64

[shawnheide]

@jreback I looked into this but am confused as to how to implement the cross-linking in the docstrings. The expanding function for dataframes and series don't have a doc page for .sum, .product, etc. As I understand it this is just chaining the .expanding method with the .sum method, so there shouldn't be a separate doc page right?

As an alternative we could just cross-link .expanding to all of the .cum* and vice-versa. But I'm not sure if that's what you were going for.

[jreback]

just show an example as indicated above