doc: explain pgstatindex fragmentation

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 <frederic.yhuel@dalibo.com>
Author: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Benoit Lobréau <benoit.lobreau@dalibo.com>
Discussion: https://postgr.es/m/bf110561-f774-4957-a890-bb6fab6804e0%40dalibo.com
Discussion: https://postgr.es/m/4c5dee3a-8381-4e0f-b882-d1bd950e8972@dalibo.com