pgsql: doc: clarify that max_wal_size is "during" checkpoints

doc: clarify that max_wal_size is "during" checkpoints

Previous wording was "between".

Reported-by: Pavel Luzanov

Discussion: /messages/by-id/26906a54-d7cb-2f8e-eed7-e31660024694@postgrespro.ru

Backpatch-through: 9.5

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/db864c3c36035e4620afd114c783af7d777d78b0

Modified Files
--------------
doc/src/sgml/config.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)