Minor cleanup/code review for "indirect toast" stuff.

 Minor cleanup/code review for "indirect toast" stuff.
 
 Fix some issues I noticed while fooling with an extension to allow an
 additional kind of toast pointer.  Much of this is just comment
 improvement, but there are a couple of actual bugs, which might or might
 not be reachable today depending on what can happen during logical
 decoding.  An example is that toast_flatten_tuple() failed to cover the
 possibility of an indirection pointer in its input.  Back-patch to 9.4
 just in case that is reachable now.
 
 In HEAD, also correct some really minor issues with recent compression
 reorganization, such as dangerously underparenthesized macros.