Add a bit more about polarization

Author: apragsdale

docs/stats.md

@@ -702,9 +702,13 @@ similarly to most other functions with respect to `sample_sets` and `indexes`.
 
 Two-locus statistics can be computed using two modes, either `site` or
 `branch`, and these should be interpreted in the same way as these modes in the
-single-site statistics. Within this framework, we also implement polarisation,
-but do not expose it to the user, opting to provide statistics that are
-polarised where appropriate.
+single-site statistics. Within this framework, statistics may be either
+polarised or unpolarised. For statistics that are polarised, we compute
+statistic values for pairs of derived alleles. Unpolarised statistics compute
+statistics over all possible alleles, derived and ancestral, and the result in
+averaged over statistics computed for all pairs of alleles (see weighting
+schemes below). The option for polarisation is not exposed to the user.
+Instead, we implement statistics that are polarised where appropriate.
 
 (sec_stats_two_locus_site)=