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 <[email protected]>
Author: Laurenz Albe <[email protected]>
Reviewed-by: Bertrand Drouvot <[email protected]>
Reviewed-by: Benoit Lobréau <[email protected]>
Discussion: https://postgr.es/m/bf110561-f774-4957-a890-bb6fab6804e0%40dalibo.com
Discussion: https://postgr.es/m/4c5dee3a-8381-4e0f-b882-d1bd950e8972@dalibo.com