Update from-into.md (#245)

Explaining how those traits are used in practice and when you should use one over another.

Author: igor-petruk

src/traits/from-into.md

@@ -23,3 +23,11 @@ fn main() {
     println!("{s}, {addr}, {one}, {bigger}");
 }
 ```
+
+<details>
+  
+* That's why it is common to only implement `From`, as your type will get `Into` implementation too.
+* When declaring a function argument input type like "anything that can be converted into a `String`", the rule is opposite, you should use `Into`.
+  Your function will accept types that implement `From` and those that _only_ implement `Into`.
+    
+</details>