Editorial overhaul of plpgsql documentation. Provide detailed documentation

of variable substitution and plan caching behavior in dedicated sections.
(A lot of this material existed already, but was scattered in various places
in the chapter.)  Reorganize material a little bit, mostly to try to avoid
diving into deep details in the first introductory sections.  Document some
fine points that had escaped treatment before, notably the ability to qualify
plpgsql variable names with block labels.  Some minor wordsmithing here and
there.