Improve writing of designation proposal again.

Remove term "target object" from s8.5 and made it a generic term in
s8.5.1.  Remove mentions of (nth-order) initialization lists, elision
order, and designated-follower. Replace the text defining them with new
text at new places for describing an aggregate's applicable sub-objects
and for determining targeted sub-objects of initializer clauses with or
without designators.  Update other paragraphs of s8.5.1 to deal with
the changed descriptions.  Give expanded reasoning on removing s8.5.1/8
and /15. Add an adjusted version of s8.5.1/9.  Do some other tweaks.