fix linked iterator discussion

[justinmeiners]

• push_back, push_front, etc are never discussed for linked list iterators. I left them in the code, but added a comment to explain this.
• the code example test_list_pool_iterator.cpp creates lists without using that functionality.
• Add a bit of exposition from Alex left out.
• Fix typos.
• clarify exercise and where it is solved.

[justinmeiners]

Thanks, I wrote this late at night and figured it would be a disaster.