[docs] add comment for rust::Str size and length

The presence of `size` and `length` confused me, adding a small comment to make it more clear for future readers.

Author: kassens

book/src/binding/str.md

@@ -28,7 +28,9 @@ public:
 
   // Note: no null terminator.
   const char *data() const noexcept;
+  // Length in bytes
   size_t size() const noexcept;
+  // Length in bytes (alias for `size()`)
   size_t length() const noexcept;
   bool empty() const noexcept;