[DOC] Fix for the missing pg_stat_progress_cluster view phase column value

[Shinoda, Noriyoshi (PN Japan A&PS Delivery)]

Hi, 

I found a missing column value in the pg_stat_progress_cluster view document.
I read the src/backend/catalog/system_views.sql file, there seems to be a 
possibility that 'writing new heap' is output in the 'phase' column.
The attached patch adds a description of the 'writing new heap' value output in 
the 'phase' column.

Regards,
Noriyoshi Shinoda

monitoring_sgml_v1.diff
Description: monitoring_sgml_v1.diff