Rollup merge of #22027 - iblech:patch-1, r=steveklabnik

Rollup merge of #22027 - iblech:patch-1, r=steveklabnik

The first commit adds a short note which I believe will reduce worries in people who work with closures very often and read the Rust book for their first time.

The second commit consists solely of tiny typo fixes. In some cases, I changed "logical" quotations like

    She said, "I like programming".

to

    She said, "I like programming."

because the latter seems to be the prevalent style in the book.