Docs: impls of PartialEq/PartialOrd/Ord must agree
Fixes#41270.
This PR brings two improvements to the docs:
1. Docs for `PartialEq`, `PartialOrd`, and `Ord` clarify that their implementations must agree.
2. Fixes a subtle bug in the Dijkstra example for `BinaryHeap`, where the impls are inconsistent.
Thanks @Rufflewind for spotting the bug!
r? @alexcrichton
cc @frankmcsherry