Doc: updates for v12 release notes.

Remove mention of ECPG's DECLARE STATEMENT, since that was reverted
yesterday.  Rewrite some other entries per suggestions from Peter
Eisentraut.  Make a couple of desultory wording and markup adjustments.

Discussion: https://postgr.es/m/d4dff575-90ab-9c8c-cc6f-8c657e2de665@2ndquadrant.com