The documentation for sharing a mutable state did not provide examples.
Wrote explanation of the steps required
to share mutable states between routes.

Provided an example in code.

This was inspired from discussion tokio-rs#629
tokio-rs#629

The cost of using an async mutex was not mentioned.
Rather than defaulting to async, the documentation
now gives the std Mutex in the example and links
to the discussion from tokio.

Example was opinionated and specific.
Also, changed written description to be more general.

Provides more consistent style across code base

Vagelis had a similar pull request. Changes were merged.
Added a note about deadlocks. A detailed discussion may not be appropriate.
However, a note that makes people aware of the potential consequences could help reduce pitfalls of share mutable state.

Co-authored-by: Jonas Platte <jplatte+git@posteo.de>

Co-authored-by: Jonas Platte <jplatte+git@posteo.de>