Rollup merge of rust-lang#36997 - KillTheMule:patch-1, r=steveklabnik

Book: Be very explicit of lifetimes being descriptive

... not prescriptive. Pointed out in https://users.rust-lang.org/t/what-if-i-get-lifetimes-wrong/7535/4, which was a revelation to me and made me think this should be more clear in the book. I'm not sure if I got this entirely right or if the wording is good, but I figured a PR is more helpful than a simple issue.

r? @steveklabnik

Small Note: There's also https://github.com/rust-lang/book, should I have sent the PR there? It doesn't coincide with the online book though, so I figured it's better of here.

src/doc/book/lifetimes.md

@@ -56,8 +56,12 @@ To fix this, we have to make sure that step four never happens after step
 three. The ownership system in Rust does this through a concept called
 lifetimes, which describe the scope that a reference is valid for.
 
-When we have a function that takes an argument by reference, we can be
-implicit or explicit about the lifetime of the reference:
+**Note** It's important to understand that lifetime annotations are
+_descriptive_, not _prescriptive_. This means that how long a reference is valid
+is determined by the code, not by the annotations. The annotations, however,
+give information about lifetimes to the compiler that uses them to check the
+validity of references. The compiler can do so without annotations in simple
+cases, but needs the programmers support in complex scenarios.
 
 ```rust
 // implicit