Apply suggestions from code review

Co-authored-by: Ryan Scheel <[email protected]>

Author: RalfJung

src/behavior-considered-undefined.md

@@ -57,9 +57,9 @@ code.
     > **Note**: `rustc` achieves this with the unstable
     > `rustc_layout_scalar_valid_range_*` attributes.
 
-Note that uninitialized memory is also implicitly invalid for any type that has
-a restricted set of valid values.  In other words, the only cases in which
-reading uninitialized memory is permitted is inside `union`s, and between the
+**Note:** Uninitialized memory is also implicitly invalid for any type that has
+a restricted set of valid values. In other words, the only cases in which
+reading uninitialized memory is permitted is inside `union`s and between the
 fields of a compound type (in the "padding").
 
 A reference/pointer is "dangling" if it is null or not all of the bytes it