core::iter::Unfold: reword docs and add example

Added the example from [this Reddit thread][1], reworked to be more robust with correct logic (first link skipped the 0th and 1st Fibonacci numbers, second forgot about the last two valid values before overflow). Will yield all Fibonacci numbers sequentially in the range `[0, <u32 as Int>::max_value())`.

If the example is too complicated I can change it to a more naive version, perhaps using signed integers to check for overflow instead of `Option` and `.checked_add()`.

Also reworded the doc comments to clarify the usage and behavior of `Unfold`, as the thread suggested that it wasn't really clear how `Unfold` worked and when one should use it.

This change is in the `core` crate but I based the example on `std` since that's where most readers will find the example. I included a note about `core` for clarity.

Tested with `rustdoc src/libcore/lib.rs`. Rebased against latest master as of the creation of this PR.

[1]: http://www.reddit.com/r/rust/comments/2ny8r1/a_question_about_loops/cmighu4?context=10000
We don't need to say this, as a ton of stuff is just reexported from core. The current approach is to just treat all inline docs as though they're just in `std`.