Rollup merge of #33335 - cramertj:master, r=alexcrichton

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

Current documentation for `std::mem::size_of` is ambiguous, and the documentation for `std::intrinsics::size_of` incorrectly defines size.

This fix re-defines size as the offset in bytes between successive instances of a type, as described in LLVM's [getTypeAllocSize](http://llvm.org/docs/doxygen/html/classllvm_1_1DataLayout.html#a1d6fcc02e91ba24510aba42660c90e29).

Fixes: #33266