Add examples, more documentation around health checks

Problem description

Healthchecks are a powerful feature, but the documentation is currently sparse. There is some basic information on its usage in the builder documentation, and the flags are included in the docker container run and docker service create command line reference, but no practical examples are included in the documentation.

Suggestions for a fix

Docker 17.05 and up adds additional options for health checks (start period), and I wrote a comment on that pull request to illustrate the functionality which may be useful as a starting point https://github.com/moby/moby/pull/28938#issuecomment-301736698, https://github.com/moby/moby/issues/35881#issuecomment-356619230, and I saw @tomwillfixit has a demo repository to show health checks (https://github.com/tomwillfixit/healthcheck).

Those examples are very basic, but can be useful in itself (possibly we may want to add a "real" example as well).

/cc @mstanleyjones

also /cc @dongluochen @tianon who may be able to provide more information on this subject 👍

Start with docs/reference/builder.md, docs/reference/commandline/run.md, and docs/reference/commandline/service_create.md, then review the linked health-check examples and demo repository. Document practical health-check usage, including the newer start-period option, and consider whether a more realistic example is needed. Done means the relevant references contain useful examples beyond the existing flag descriptions.