doc: Mention that partitions_{done,total} is 0 for REINDEX progress reports

 doc: Mention that partitions_{done,total} is 0 for REINDEX progress reports
 
 REINDEX has recently gained support for partitions, so it can be
 confusing to see those fields not being set.  Making useful reports for
 for such relations is more complicated than it looks with the current
 set of columns available in pg_stat_progress_create_index, and this
 touches equally REINDEX DATABASE/SYSTEM/SCHEMA.  This commit documents
 that those two columns are not touched during a REINDEX.
 
 Reported-by: Justin Pryzby
 Discussion: https://postgr.es/m/20210216064214.GI28165@telsasoft.com