Improve the description of the ["chain cover index"](https://matrix-org.github.io/synapse/latest/auth_chain_difference_algorithm.html) used internally by Synapse.
> in: 2008 IEEE 24th International Conference on Data Engineering, April 2008,
> pp. 893–902. (PDF available via [Google Scholar](https://scholar.google.com/scholar?q=Y.%20Chen,%20Y.%20Chen,%20An%20efficient%20algorithm%20for%20answering%20graph%20reachability%20queries,%20in:%202008%20IEEE%2024th%20International%20Conference%20on%20Data%20Engineering,%20April%202008,%20pp.%20893902.).)
for a more modern take.
In practical terms, the chain cover assigns every event a
*chain ID* and *sequence number* (e.g. `(5,3)`), and maintains a map of *links*
between events in chains (e.g. `(5,3) -> (2,4)`) such that `A` is reachable by `B`
(i.e. `A` is in the auth chain of `B`) if and only if either:
1. `A` and `B` have the same chain ID and `A`'s sequence number is less than `B`'s
sequence number; or
2. there is a link `L` between `B`'s chain ID and `A`'s chain ID such that
`L.start_seq_no` <= `B.seq_no` and `A.seq_no` <= `L.end_seq_no`.
@@ -49,8 +81,9 @@ There are actually two potential implementations, one where we store links from
each chain to every other reachable chain (the transitive closure of the links
graph), and one where we remove redundant links (the transitive reduction of the
links graph) e.g. if we have chains `C3 -> C2 -> C1` then the link `C3 -> C1`
would not be stored. Synapse uses the former implementations so that it doesn't
need to recurse to test reachability between chains.
would not be stored. Synapse uses the former implementation so that it doesn't
need to recurse to test reachability between chains. This trades-off extra storage