separate safe/unsafe functions in challenge 22

Author: thanhnguyen-aws

doc/src/challenges/0022-str-iter.md

@@ -22,7 +22,7 @@ Verify the safety of [`std::str`] functions that are defined in (library/core/sr
 
 ### Success Criteria
 
-Write and prove the contract for the safety of the following functions:
+Prove the safety of the following safe functions that contain unsafe code:
 
 
 | Function | Impl for |
@@ -43,6 +43,8 @@ Write and prove the contract for the safety of the following functions:
 |next_back| MatchesInternal|
 |remainder| SplitAsciiWhitespace|
 
+Write and prove the contract for this unsafe function: __iterator_get_unchecked
+
 The verification must be unbounded---it must hold for str of arbitrary length.