Clarify block_reduce func keyword parameter [ci skip]
Clarify block_reduce func keyword parameter [ci skip]

mhvk/astropy,pllim/astropy,astropy/astropy,larrybradley/astropy,mhvk/astropy,astropy/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,astropy/astropy,lpsinger/astropy,pllim/astropy,mhvk/astropy,lpsinger/astropy,mhvk/astropy,lpsinger/astropy,larrybradley/astropy,larrybradley/astropy,mhvk/astropy,astropy/astropy,lpsinger/astropy,larrybradley/astropy,pllim/astropy,astropy/astropy,pllim/astropy