Clarify unit of size in docs for size_of functions.
Changed the docs in mem.rs to clarify the fact the the size functions return sizes in bytes.
Regression test closes #7660
auto merge of #12374 : dylanbraithwaite/rust/size_of_unit_clarification, r=cmr

Changed the docs for std::mem to clarify the fact that the size functions return sizes in bytes.
auto merge of #12375 : FlaPer87/rust/issue-7660, r=cmr