doc: Add description about re-analysis and re-planning of a prepared statement.

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

doc/src/sgml/ref/prepare.sgml

@@ -184,7 +184,8 @@ EXPLAIN EXECUTE <replaceable>name</replaceable>(<replaceable>parameter_values</r
    analysis and planning of the statement, <productname>PostgreSQL</productname> will
    force re-analysis and re-planning of the statement before using it
    whenever database objects used in the statement have undergone
-   definitional (DDL) changes since the previous use of the prepared
+   definitional (DDL) changes or the planner statistics of them have
+   been updated since the previous use of the prepared
    statement.  Also, if the value of <xref linkend="guc-search-path"/> changes
    from one use to the next, the statement will be re-parsed using the new
    <varname>search_path</varname>.  (This latter behavior is new as of