Added an Alternative way for printing struct and declaring a string. (#1773)

Knowing to_string method is good for learner to use at some situation. 
Knowing print complete struct object can be useful while debug.

---------

Co-authored-by: Dustin J. Mitchell <djmitche@google.com>

src/std-types/string.md

@@ -54,5 +54,10 @@ all `str` methods on a `String`.
     out-of-bounds.
   - To a substring by using `s3[0..4]`, where that slice is on character
     boundaries or not.
+- Many types can be converted to a string with the
+  [`to_string`](https://doc.rust-lang.org/std/string/trait.ToString.html#tymethod.to_string)
+  method. This trait is automatically implemented for all types that implement
+  `Display`, so anything that can be formatted can also be converted to a
+  string.
 
 </details>

src/types-and-values/strings.md

@@ -53,4 +53,7 @@ later, but this is enough for subsequent slides and exercises to use strings.
   }
   ```
 
+- Using `{:?}` is a convenient way to print array/vector/struct of values for
+  debugging purposes, and it's commonly used in code.
+
 </details>