Add more documentation to Register.
Add more documentation to Register.

This adds documentation about how using the Register function causes the Any
value to be treated as JSON. I'd found this confusing when new to the package.

Also, add a few periods to function docs to make things consistent.

Signed-off-by: Seth Bunce <72bff4150c530f716f049485b33db27b61cbaaa9@getcruise.com>

containerd/typeurl