doc: explain pgstatindex fragmentation

It was quite hard to guess what leaf_fragmentation meant without looking
at pgstattuple's code.  This patch aims to give to the user a better
idea of what it means.

Author: Frédéric Yhuel
Author: Laurenz Albe
Reviewed-by: Bertrand Drouvot, Benoît Lobréau
Discussion: https://postgr.es/m/bf110561-f774-4957-a890-bb6fab6804e0%40dalibo.com
Discussion: https://postgr.es/m/4c5dee3a-8381-4e0f-b882-d1bd950e8972@dalibo.com

Author: laurenz

doc/src/sgml/pgstattuple.sgml

@@ -270,6 +270,15 @@ leaf_fragmentation | 0
      page than is accounted for by <literal>internal_pages + leaf_pages +
      empty_pages + deleted_pages</literal>, because it also includes the
      index's metapage.
+     <literal>avg_leaf_density</literal> is the fraction of the index size that
+     is taken up by user data.  Since indexes have a default fillfactor of 90,
+     this should be around 90 for newly built indexes of non-negligible size,
+     but usually deteriorates over time.
+     <literal>leaf_fragmentation</literal> represents a measure of disorder.
+     A higher <literal>leaf_fragmentation</literal> indicates that the
+     physical order of the index leaf pages increasingly deviates from their
+     logical order. This can have a significant impact if a large part
+     of the index is read from disk.
     </para>
 
     <para>