Clarify what Behavior Considered Undefined means.
Fixes
Thanks to for providing the initial sentence.
## Behavior considered undefined
-The following is a list of behavior which is forbidden in all Rust code,
-including within `unsafe` blocks and `unsafe` functions. Type checking provides
-the guarantee that these issues are never caused by safe code.
+Rust code, including within `unsafe` blocks and `unsafe` functions is incorrect
+if it exhibits any of the behaviors in the following list. It is the
+programmer's responsibility when writing `unsafe` code that it is not possible
+to let `safe` code exhibit these behaviors.
* Data races.
* Dereferencing a null or dangling raw pointer.
