Add note about reversible mutations creating invariant sites

Author: apragsdale

docs/stats.md

@@ -886,7 +886,10 @@ print(ld_sub)
 
 Any remaining sites that produce {math}`r^2` with a nan values are those with
 a mutation that occurs above the root of the tree, so that all samples in the
-subset carry the derived allele (so that its allele frequency is 1).
+subset carry the derived allele (so that its allele frequency is 1). Also note
+that under certain mutation models, back mutations may create sites that are
+invariant in a sample, and this will also result in nan values observed in the
+LD matrix.
 
 (sec_stats_two_locus_sample_one_way_stats)=