book: explanation that String -> &str coercion doesn't happen for &str traits

Author: thombles

src/doc/trpl/strings.md

@@ -49,6 +49,18 @@ fn main() {
 }
 ```
 
+This coercion does not happen for functions that accept one of `&str`’s traits
+instead of `&str`. For example, [`TcpStream::connect`][connect] has a parameter
+of type `ToSocketAddrs`. A `&str` is okay but a `String` must be explicitly
+converted using `&*`.
+
+```rust
+TcpStream::connect("192.168.0.1:3000"); // &str parameter
+
+let addr_string = "192.168.0.1:3000".to_string();
+TcpStream::connect(&*addr_string); // convert addr_string to &str
+```
+
 Viewing a `String` as a `&str` is cheap, but converting the `&str` to a
 `String` involves allocating memory. No reason to do that unless you have to!
 
@@ -127,3 +139,4 @@ This is because `&String` can automatically coerce to a `&str`. This is a
 feature called ‘[`Deref` coercions][dc]’.
 
 [dc]: deref-coercions.html
+[connect]: ../std/net/struct.TcpStream.html#method.connect