Add requirements of TotalEq and TotalOrd

Clarify that TotalEq needs an underlying equivalence relation and that TotalOrd
needs a total ordering and specifically named the required (and sufficient)
attributes.
Remove use of block comments in src/libstd/cmp.rs
auto merge of #13358 : tbu-/rust/pr_doc_equivrel, r=cmr

Add requirements of TotalEq and TotalOrd

Clarify that TotalEq needs an underlying equivalence relation and that TotalOrd
needs a total ordering and specifically named the required (and sufficient)
attributes.