Documentation review for Boost.Coroutine feature/symmetric branch.

Many small grammar tweaks, probably applicable to master branch too.

The one substantive comment specifically about feature/symmetric: can we get
away without yield_type::operator()?

I think there may be a bug in the symmetric_coroutine merge example. I've changed the final copy-rest-of-elements loop. I've also propagated the comments in one instance to both instances.

Can the specified stack size actually be equal to either minimum_stacksize() or maximum_stacksize(), or must it be strictly between them? Also, in a couple of preconditions, I think the sense of the comparison was misstated.

I've added remarks and questions flagged with "Oliver:" in the documentation.