minor frollmean doc improvements (#3240)
minor frollmean doc improvements (#3240)


Rdatatable/data.table,vlulla/data.table,jangorecki/data.table,vlulla/data.table,jangorecki/data.table,Rdatatable/data.table,jangorecki/data.table,Rdatatable/data.table,vlulla/data.table,vlulla/data.table,Rdatatable/data.table,jangorecki/data.table