doc: Mention de-normalization of deallocated entries in pg_stat_statements

doc: Mention de-normalization of deallocated entries in pg_stat_statements

The current implementation of query normalization in pg_stat_statements
is optimistic.  If an entry is deallocated between the post-analyze hook
and the planner and/or execution hook, it can be possible to find query
strings with literal constant values (like "SELECT 1, 2") rather than
their normalized flavor (like "SELECT $1, $2").

This commit adds in the documentation a paragraph about this limitation,
and that this risk can be reduced by increasing pg_stat_statements.max,
particularly if pg_stat_statements_info reports a high number of
deallocations.

Author: Sami Imseih
Discussion: https://postgr.es/m/9CFF3512-355B-4676-8CCC-6CF622F4DC1A@amazon.com