finish the documentation for implicit casting

[andrewrk]

Moved from #367

The docs have a bunch of TODOs:

Implicit Cast: Arrays

TODO: [N]T to []const T

TODO: *const [N]T to []const T

TODO: [N]T to *const []const T

TODO: [N]T to ?[]const T

TODO: *[N]T to []T

TODO: *[N]T to [*]T

TODO: *[N]T to ?[*]T

TODO: *T to *[1]T

TODO: [N]T to E![]const T
Implicit Cast: Optionals

TODO: T to ?T

TODO: T to E!?T

TODO: null to ?T
Implicit Cast: T to E!T

TODO
Implicit Cast: E to E!T

TODO
Implicit Cast: comptime_int to *const integer

TODO
Implicit Cast: comptime_float to *const float

TODO
Implicit Cast: compile-time known numbers

TODO
Implicit Cast: union to enum

TODO
Implicit Cast: enum to union

TODO
Implicit Cast: T to *T when @sizeOf(T) == 0

TODO
Implicit Cast: undefined

TODO
Implicit Cast: T to *const T

TODO