A prepared statement is re-analyzed and re-planned whenever database
objects used in the statement have undergone definitional changes or
the planner statistics of them have been updated. The former has been
documented from before, but the latter was not previously. This commit
adds the description about the latter case into the docs.

Author: Atsushi Torikoshi
Reviewed-by: Andy Fan, Fujii Masao
Discussion: https://postgr.es/m/3ac82f4817c9fe274a905c8a38d87bd9@oss.nttdata.com