The milestones marking California’s wettest year in decades continued to pile up Thursday, as state water officials said a reservoir high up in the Sierra Nevada has exceeded capacity for the first time in 21 years. Lake Davis began overflowing onto its earth-and-rock spillway Wednesday after a couple of light rainstorms this week, Department of Water Resources officials said. “While DWR does not anticipate problems downstream of the reservoir near Portola, flows below the lake could exceed what residents, businesses and anglers have experienced over the past three decades,” the agency said Thursday.