doc: Mention possible ephemeral discrepancies in pg_stat_activity

doc: Mention possible ephemeral discrepancies in pg_stat_activity

Ephemeral inconsistencies across multiple attributes of pg_stat_activity
can exist as the system is designed to be efficient with a low overhead.
This question is raised by users from time to time based on the data
read in the view, so let's add a note in the docs about this
possibility.

Author: Alex Friedman 
Reviewed-by: Sami Imseih 
Discussion: https://postgr.es/m/8a275154-a654-44b0-ab37-197802f04c7b@gmail.com