Clarify Boot Image Version usage and example

[bramsdell-ms]

I can submit a PR for this, but in the rt685s example, it uses the crate version as the Boot Image Version. Based on my understanding, this needs to be well-formed according to the reference manual (something about two bytes for the version, the other two bytes is the complement of the version)

I found this out the hard way when I changed the version in my project.

I can update the example, but I can't test it. But the short answer I think is "unless you know what you're doing, don't touch this."