Safety pamphlet: More details in first section "Unsafe Code"

[pellico]

I would suggest the following:

• explain with more details the relationship between "unsafe code" and "undefined behavior".
• clarify that the definition of undefined behavior is evolving and not complete as stated in Rust Reference
• state that in safety critical application at the beginning of the concept the definition of "undefined behavior" and consequently the usage of "unsafe" keyword shall be fully specified and any changes shall be tracked and evaluated. (I am not sure the pamphlet is the right place to formally state rule but it worth to mention the concept)

[adfernandes]

@PLeVasseur please assign to me