Workflow-init support docs

[drewhoskins-temporal]

We should have a method that's guaranteed to initialize before signals and updates come in so handlers can use initialized values.

[Quinn-With-Two-Ns]

@cretz Can we close this issue? I don't think Go SDK needs any changes here if we want to support struct based workflows or improve docs I think those should be tracked separately

[cretz]

I do think we can, let's confirm with team first though (same for TypeScript). Starting internal discussion...