Int64#parse* method docs should declare what exceptions they throw

[trinarytree]

Int64#parse{Hex, Int, Radix} apparently throw FormatException. this should be declared in their docs so that callers can narrow a surrounding try-catch to just the correct exception type.

btw, i initially file this issue here https://github.com/dart-lang/sdk/issues/32383 but was told this is a more appropriate place.