docs: Changed docs for `size_of` to describe size as a stride offset

docs: Changed docs for `size_of` to describe size as a stride offset
Current description of `std::mem::size_of` is ambiguous, and the
`std::intrinsics::size_of` description incorrectly defines size
as the number of bytes necessary to exactly overwrite a value,
not including the padding between elements necessary in a vector
or structure.