Document append+overwrite arg combo for HDF5
Document append+overwrite arg combo for HDF5

See #2624

astropy/astropy,StuartLittlefair/astropy,bsipocz/astropy,DougBurke/astropy,joergdietrich/astropy,bsipocz/astropy,AustereCuriosity/astropy,bsipocz/astropy,dhomeier/astropy,kelle/astropy,lpsinger/astropy,lpsinger/astropy,funbaker/astropy,joergdietrich/astropy,tbabej/astropy,saimn/astropy,mhvk/astropy,funbaker/astropy,larrybradley/astropy,DougBurke/astropy,stargaser/astropy,astropy/astropy,aleksandr-bakanov/astropy,kelle/astropy,joergdietrich/astropy,lpsinger/astropy,larrybradley/astropy,astropy/astropy,pllim/astropy,lpsinger/astropy,kelle/astropy,AustereCuriosity/astropy,DougBurke/astropy,saimn/astropy,MSeifert04/astropy,MSeifert04/astropy,saimn/astropy,mhvk/astropy,funbaker/astropy,AustereCuriosity/astropy,stargaser/astropy,bsipocz/astropy,StuartLittlefair/astropy,funbaker/astropy,MSeifert04/astropy,kelle/astropy,pllim/astropy,dhomeier/astropy,pllim/astropy,tbabej/astropy,tbabej/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,joergdietrich/astropy,StuartLittlefair/astropy,larrybradley/astropy,AustereCuriosity/astropy,larrybradley/astropy,stargaser/astropy,saimn/astropy,astropy/astropy,stargaser/astropy,mhvk/astropy,AustereCuriosity/astropy,saimn/astropy,mhvk/astropy,StuartLittlefair/astropy,kelle/astropy,tbabej/astropy,pllim/astropy,mhvk/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,lpsinger/astropy,MSeifert04/astropy,pllim/astropy,larrybradley/astropy,tbabej/astropy,joergdietrich/astropy,DougBurke/astropy,astropy/astropy,StuartLittlefair/astropy