Rollup merge of #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.