Improve docs for collecting into `Option`s
Improve docs for collecting into `Option`s

Changes the original example to use more idiomatic formatting as well
as `.checked_add`. Also adds a second example to show a case where the
`.collect` returns `None`.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust