Update 'Strings' chapter of the book

Fix #29823 by further explaining `&str` and pointing out the
difference between `&str` and `&'static str`.

Author: JanLikar

src/doc/trpl/strings.md

@@ -12,17 +12,17 @@ encoding of UTF-8 sequences. Additionally, unlike some systems languages,
 strings are not null-terminated and can contain null bytes.
 
 Rust has two main types of strings: `&str` and `String`. Let’s talk about
-`&str` first. These are called ‘string slices’. String literals are of the type
-`&'static str`:
+`&str` first. These are called ‘string slices’. A string slice has a fixed
+size, and cannot be mutated. It is a reference to a sequence of UTF-8 bytes.
 
 ```rust
 let greeting = "Hello there."; // greeting: &'static str
 ```
 
-This string is statically allocated, meaning that it’s saved inside our
-compiled program, and exists for the entire duration it runs. The `greeting`
-binding is a reference to this statically allocated string. String slices
-have a fixed size, and cannot be mutated.
+`"Hello there."` is a string literal and its type is `&'static str`. String
+literal is a string slice that is statically allocated, meaning that it’s saved
+inside our compiled program, and exists for the entire duration it runs. The
+`greeting` binding is a reference to this statically allocated string.
 
 String literals can span multiple lines. There are two forms. The first will
 include the newline and the leading spaces: