Document that Tracer#extract can return null

[realark]

See #168

#203

There are two proposed changes in the above issue:

• Documenting that Tracer#extract can return null (many tracer impls are already doing this)
• Adding @Nullable annotation to extract to assist static checkers

This PR only addresses the doc change. The annotation will be proposed in a separate PR.

[coveralls]

Coverage remained the same at 73.368% when pulling 68d5b30 on realark:ark/nullable-tracer-extract into 4a04d98 on opentracing:master.