DOC: Copy-edit the 'partition' docstring.
DOC: Copy-edit the 'partition' docstring.

Closes gh-7958.

numpy/numpy,mhvk/numpy,mhvk/numpy,endolith/numpy,endolith/numpy,charris/numpy,charris/numpy,numpy/numpy,mattip/numpy,mhvk/numpy,charris/numpy,numpy/numpy,mattip/numpy,charris/numpy,numpy/numpy,mhvk/numpy,mattip/numpy,endolith/numpy,endolith/numpy,mattip/numpy,mhvk/numpy