[doc] plan invalidation when statistics are update

Hi,

AFAIU, when the planner statistics are updated, generic plans are 
invalidated and PostgreSQL recreates. However, the manual doesn't seem 
to explain it explicitly.

   https://www.postgresql.org/docs/devel/sql-prepare.html

I guess this case is included in 'whenever database objects used in the 
statement have definitional (DDL) changes undergone', but I feel it's 
hard to infer.

Since updates of the statistics can often happen, how about describing 
this case explicitly like an attached patch?


Regards,

--
Atsushi Torikoshi