Clarify that str data must still be initialized

src/what-unsafe-does.md

@@ -30,7 +30,7 @@ language cares about is preventing the following things:
     * a `char` outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
     * a `!` (all values are invalid for this type)
     * an integer (`i*`/`u*`), floating point value (`f*`), or raw pointer read from
-      [uninitialized memory][]
+      [uninitialized memory][], or uninitialized memory in a `str`.
     * a reference/`Box` that is dangling, unaligned, or points to an invalid value.
     * a wide reference, `Box`, or raw pointer that has invalid metadata:
         * `dyn Trait` metadata is invalid if it is not a pointer to a vtable for
@@ -48,11 +48,11 @@ A reference/pointer is "dangling" if it is null or not all of the bytes it
 points to are part of the same allocation (so in particular they all have to be
 part of *some* allocation). The span of bytes it points to is determined by the
 pointer value and the size of the pointee type. As a consequence, if the span is
-empty, "dangling" is the same as "non-null". Note that slices point to their
+empty, "dangling" is the same as "non-null". Note that slices and strings point
-entire range, so it's important that the length metadata is never too large
+to their entire range, so it's important that the length metadata is never too
-(in particular, allocations and therefore slices cannot be bigger than
+large (in particular, allocations and therefore slices and strings cannot be
-`isize::MAX` bytes). If for some reason this is too cumbersome, consider using
+bigger than `isize::MAX` bytes). If for some reason this is too cumbersome,
-raw pointers.
+consider using raw pointers.
 
 That's it. That's all the causes of Undefined Behavior baked into Rust. Of
 course, unsafe functions and traits are free to declare arbitrary other