docs: update start docs to add examples

[craig-osterhout]

- What I did
There are no examples in the documentation for the docker start options.
I created examples in the documentation for every option.

- How I did it
I created examples based on examples from related commands. Then, verified that they worked.
For 3 options, I was unable to verify the results. No CLI errors were given using the example syntax. I added them assuming it was not a CLI syntax issue.

• For --checkpoint, when I run docker start, it just hangs. On Linux with experimental enabled and criu installed.
• For --checkpoint-dir, it gives a message that it's not supported. Already a moby issue opened regarding this.
• For --detach-keys, I was unable to get it to work on Windows or Linux.

- How to verify it
Run the examples.

- Description for the changelog
Updated start docs to add examples

[codecov-commenter]

Codecov Report

Merging #4318 (22aff71) into master (5be2139) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@ Coverage Diff @@## master #4318 +/- ##
=======================================
Coverage 59.07% 59.07% =======================================
Files 287 287 Lines 24752 24752 =======================================
Hits 14623 14623 Misses 9249 9249 Partials 880 880 

[thaJeztah]

Hm.. I thought we had a /go/ redirect for this one, but looks like that's only for experimental CLI features (but those are now enabled by default). https://github.com/docker/docs/blob/8d81583dcf89cf302420ca5f5c8a39b22a7e73fb/_redirects.yml#L106-L107

Perhaps we should repurpose that redirect for both cli (no longer needed) and engine features (and have a canonical section about enabling experimental features, etc).

@dvdksn perhaps you have ideas on that?