Polish message passing feature development guide

[drewhoskins-temporal]

What does this PR do?

• More concrete, evocative use cases.
• Highlight overview guide in front of per-SDK docs--I suspect it's more likely to be the next place to go for new users.
• Fix some out-of-date Update messaging.
• Polish

Notes to reviewers

Tested with

yarn start
http://localhost:3000/evaluate/development-production-features/workflow-message-passing