Improve comment of DeallocateStmt->isall

Improve comment of DeallocateStmt->isall

This field is not used directly in the code, but it is important for
query jumbling to be able to make a difference between a named
DEALLOCATE and DEALLOCATE ALL (see bb45156f342c).  This behavior is
tracked in the regression tests of pg_stat_statements, but the reason
why this field is important can be easily missed, as a recent discussion
has proved, so let's improve its comment to document the reason why it
needs to be around.

Wording has been suggested by Tom Lane

Discussion: https://postgr.es/m/Zih1ATt37YFda8_p@paquier.xyz