DOCS: Mistake in .cum* descriptions

[shawnheide]

For the .cum* functions the generated documentation description is redundant. As an example, DataFrame.cumsum() has the description "Return cumulative cumsum over requested axis." when it should say "Return cumulative sum over requested axis.".

After looking into this, I found the problem on line 5507 of generic.py where the name is being used instead of the desc.