Docs for `Single` needs an example

[viridia]

How can Bevy's documentation be improved?

It took me a while to figure out that I needed to call .into_inner() to get the content out. I kind of assumed that it would be like other queries. A short example would help.

[jpaugh]

Another useful example:

• You can call methods on the contained item directly, since Single implements Deref/DerefMut.