feat(conversions): add hint comments

exercises/conversions/from_str.rs

@@ -4,6 +4,8 @@
 // Additionally, upon implementing FromStr, you can use the `parse` method
 // on strings to generate an object of the implementor type.
 // You can read more about it at https://doc.rust-lang.org/std/str/trait.FromStr.html
+// Execute `rustlings hint from_str` or use the `hint` watch subcommand for a hint.
+
 use std::num::ParseIntError;
 use std::str::FromStr;
 
@@ -37,6 +39,9 @@ enum ParsePersonError {
 //    with something like `"4".parse::<usize>()`
 // 6. If while extracting the name and the age something goes wrong, an error should be returned
 // If everything goes well, then return a Result of a Person object
+//
+// As an aside: `Box<dyn Error>` implements `From<&'_ str>`. This means that if you want to return a
+// string error message, you can do so via just using return `Err("my error message".into())`.
 
 impl FromStr for Person {
     type Err = ParsePersonError;