Guide: Iterators, Macros and Unsafe wording fixes

Some more proposed wording fixes for the Guide, this time for the Iterators, Macros and Unsafe chapters. I’m not a native speaker, but this does read better to me, so please do review this critically.
Reverted to ‘are’ and rewrapped. :)

This is probably a bad place to ask, so I’ll do it anyway: what’s a good (re)wrapping policy? In general I try to make the least diff noise, but for that it would help if the line breaks were at logical places, so I rewrapped the whole paragraph at sentence breaks. I read an interesting post on this recently, but obviously can’t find the URL at the moment. Still not sure what’s a good approach.