Document reason for 2 re nodes

Fixes #2365

Author: hyanwong

docs/ancestry.md

@@ -1152,10 +1152,14 @@ as gene conversion. A separate {class}`msprime.NodeType` exists for each:
 A recombination event results in two extra nodes being recorded, one identifying
 the genome providing the genetic material to the left of the breakpoint and
 the other identifying the genome providing the genetic material to the
-right. For more information on how to set the recombination rate or specify a 
-recombination map: see {ref}`sec_ancestry_recombination`. More information on how 
-to set up a simulation with gene conversion can be found here: 
-{ref}`sec_ancestry_gene_conversion`.
+right. This makes it easy to identify the (possible multiple) segments of genome
+that travel via the left parent versus the right parent, and also means that
+so-called "diamonds" (in which lineages split and immediately rejoin) are retained
+even if edges are {meth}`squashed<tskit.EdgeTable.squash>`.
+
+For more information on how to set the recombination rate or specify a
+recombination map: see {ref}`sec_ancestry_recombination`. To set up a simulation
+with gene conversion, see {ref}`sec_ancestry_gene_conversion`.
 
 (sec_ancestry_record_migrations)=