finatra: note about not supporting JsonCreator
finatra: note about not supporting JsonCreator

Problem

The Finatra docs have stale information about JsonCreator support in
jackson-scala-module. It also recommends JsonCreator usage, but does
not actually support it.

Solution

Update docs to remove recommendations to use JsonCreator, and note that
JsonCreator is not supported.

JIRA Issues: CSL-6088

Differential Revision: https://phabricator.twitter.biz/D309724

twitter/finatra,twitter/finatra,twitter/finatra