add `col.format` eg to CDS/MRT docs [skip ci]
add `col.format` eg to CDS/MRT docs
[skip ci]

larrybradley/astropy,larrybradley/astropy,mhvk/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,astropy/astropy,StuartLittlefair/astropy,pllim/astropy,astropy/astropy,lpsinger/astropy,lpsinger/astropy,mhvk/astropy,saimn/astropy,mhvk/astropy,pllim/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,mhvk/astropy,saimn/astropy,pllim/astropy,astropy/astropy,mhvk/astropy,larrybradley/astropy,astropy/astropy,larrybradley/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,saimn/astropy,saimn/astropy,lpsinger/astropy,lpsinger/astropy