[DOC] Fix for the missing pg_stat_progress_cluster view phase columnvalue

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
inthe 'phase' column. 
The attached patch adds a description of the 'writing new heap' value output in the 'phase' column.

Regards,
Noriyoshi Shinoda