add an example of using sizeof on a struct with padding

[KristofferC]

No description provided.

[vtjnash]

Keno had proposed changing this (#46322), though we didn't finish

[KristofferC]

So is this PR fine or is it an implementation detail of what number this should report when it comes to structs with padding?

[vtjnash]

The exact number here is definitely an implementation detail. For example, on 32-bit platforms this returns:

julia> sizeof(StructWithPadding)
12

[PallHaraldsson]

Also mention the order matters for size:

julia> struct StructWithPadding_smaller
         x::Int16
         flag::UInt8
         flag2::Bool
       end

julia> struct StructWithPadding_larger
         flag::UInt8
         x::Int16
         flag2::Bool
       end

[KristofferC]

I think this is an improvement since it makes you aware the sizeof is not necessarily the sum of field sizes.