Improve formatting of comments in plpgsql.h

Improve formatting of comments in plpgsql.h

This file had some unusual comment layout.  Most of the comments
introducing structs ended up to the right of the screen and following
the start of the struct.  Some comments for struct members ended up
after the member definition.

Fix that by moving comments consistently before what they are
describing.  Also add missing struct tags where missing so that it is
easier to tell what the struct is.