Documentation fix: sum(float4) returns float4, not float8.
The old claim is from my commit
d06ebdb8d3425185d7e641d15e45908658a0177d of
2000年07月17日, but it seems to have been a plain old thinko; sum(float4) has
been distinct from sum(float8) since Berkeley days. Noted by KaiGai Kohei.
While at it, mention the existence of sum(money), which is also of
embarrassingly ancient vintage.