Doc: Improve the wording of logical slot prerequisites.

Doc: Improve the wording of logical slot prerequisites.

Replace the previous negative phrasing such as "there are no slots whose
... is not true" with a direct expression that all slots must have
conflicting = false.

Similarly, reword the requirement on the new cluster to state that it must
not have any permanent logical slots, clarifying that any existing logical
slots must have temporary set to true.

These changes improve readability without altering the meaning.

Reported-by: <mimidatabase@gmail.com>
Author: Vignesh C <vignesh21@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/177609278737.403059.14174275013090471947%40wrigleys.postgresql.org